Feature: Api framework

[k9ert]

This is a continuation of the PR #713 but moving the code to a non-master branch on my remote.
Will close the original PR. With this PR, Specter gets an API. The documentation is in the docs-folder.

The API is only activated if:

export SPECTER_API_ACTIVE=true

It's also active by default in Dev-Mode.

Apart from this new feature:

• The test-framework needed some refactoring. Better some bug-fixing. The biggest change is that the bitcoin_regtest fixture changed scope from module to session. This means that the regtest-node is only be spun up exactly once for the tests.
• Made an update of a bunch of dependecies
• One of the dependencies (cryptography) needed a newer OS-version but the used cypress-image was based on an old debian-image. So i've created a new base-image docker/cypress-base-ubuntu-focal/Dockerfile which is ubuntu based and what is from now on the new base for docker/cypress-python/Dockerfile
• The user.password is now a property which will automatically detect a plain-password and will do the necessary salted hashing
• The wallet-enpoint for creating PSBT's got a refactoring and now we have psbt_creator.py very similiar to the wallet_imported.py. There are people calling that a "command-pattern".
• The wallet_importer.py got some improvements as well
• The wallet.py has a change where we're rather throwing an error than returning 0-balances. We should have more courage to raise SpecterErrors
• I've changed the port-numbers of adhoc-bitcoind-nodes spun up in tests and i've wrapped them in try/raise to make sure that they will shutdown in any case of result of the tests (several tests).
• Finally there is also a small bugfix for this:
  k9ert, [21.06.21 14:26]

[2021-06-18 14:36:39,975] INFO in wallet_manager: Wallet already in Specter, updating wallet: testwallet_ghost
[2021-06-18 14:36:40,107] ERROR in controller: Uncaught exception: 'dict object' has no attribute 'devices_signed'
[2021-06-18 14:36:40,108] ERROR in controller: Traceback (most recent call last):
  File "/tmp/cirrus-ci-build/.env/lib/python3.8/site-packages/flask/app.py", line 1950, in full_dispatch_request
    rv = self.dispatch_request()
  File "/tmp/cirrus-ci-build/.env/lib/python3.8/site-packages/flask/app.py", line 1936, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/tmp/cirrus-ci-build/.env/lib/python3.8/site-packages/flask_login/utils.py", line 269, in decorated_view
    return func(*args, **kwargs)
  File "/tmp/cirrus-ci-build/src/cryptoadvance/specter/server_endpoints/wallets.py", line 683, in send_new
    return render_template(
  File "/tmp/cirrus-ci-build/.env/lib/python3.8/site-packages/flask/templating.py", line 137, in render_template
    return _render(
  File "/tmp/cirrus-ci-build/.env/lib/python3.8/site-packages/flask/templating.py", line 120, in _render
    rv = template.render(context)
  File "/tmp/cirrus-ci-build/.env/lib/python3.8/site-packages/jinja2/environment.py", line 1304, in render
    self.environment.handle_exception()
  File "/tmp/cirrus-ci-build/.env/lib/python3.8/site-packages/jinja2/environment.py", line 925, in handle_exception
    raise rewrite_traceback_stack(source=source)
  File "/tmp/cirrus-ci-build/src/cryptoadvance/specter/templates/wallet/send/sign/wallet_send_sign_psbt.jinja", line 2, in top-level template code
    {% set tab = 'send' %}
  File "/tmp/cirrus-ci-build/src/cryptoadvance/specter/templates/wallet/components/wallet_tab.jinja", line 1, in top-level template code
    {% extends "base.jinja" %}
  File "/tmp/cirrus-ci-build/src/cryptoadvance/specter/templates/base.jinja", line 62, in top-level template code
    {% block main %}
  File "/tmp/cirrus-ci-build/src/cryptoadvance/specter/templates/wallet/components/wallet_tab.jinja", line 12, in block 'main'
    {% block content %}
  File "/tmp/cirrus-ci-build/src/cryptoadvance/specter/templates/wallet/send/sign/wallet_send_sign_psbt.jinja", line 199, in block 'content'
    <button type="button" class="btn signing-column-btn" id="{{ device.alias }}_tx_sign_btn" {{ 'disabled style=background-color:#303c49;' if device.alias in psbt["devices_signed"] else '' }}>
jinja2.exceptions.UndefinedError: 'dict object' has no attribute 'devices_signed'
[2021-06-18 14:36:40,149] INFO in wallet_manager: Finished updating wallet:  testwallet_ghost
[2021-06-18 14:36:40,149] INFO in wallet_manager: Wallet already in Specter, updating wallet: