Add translation support to theme settings

[GinoPane]

Fields are not marked as translatable by default.
It is a theme author responsibility to configure required fields via theme's config and mark fields as translatable.

For example:

tabs:
  fields:

    ####################################################
    # Tab: Info
    ####################################################

    website_name:
      tab: Info
      label: Website Name
      type: text
      default: Octaskin
      translatable: true

And the result is:

[mjauvin]

@GinoPane I tried to test this PR using the following instance:

https://88f9e4ea8aac.octodock.com/backend

(user/pass = admin)

I have the following theme.yaml file:

name: demo
description: demo theme
author: OctoberCMS
homepage: 'https://octobercms.com'
code: ''
form:
  tabs:
    fields:
      website_name:
        tab: Info
        label: Website Name
        type: text
        default: Your website name
        translatable: true

And I get the following error when visiting the Cusomize Theme page:

Form record ID has not been specified.

[GinoPane]

@mjauvin it's really strange, now it is working, though code is the same. I tried to debug a little, commented plugin changes, loaded the page, after that un-commented changes and it worked 🤷‍♂

[mjauvin]

Well, you probably did something that got cached or something. I created a new instance, added the same theme.yaml and I get the same error again:

https://cfc0174a7a29.octodock.com/backend

[GinoPane]

@mjauvin Hm, it seems to be happening when cms_theme_data record is missing for the theme. For any reason the system cannot create the record.

It's interesting 🤔

[mjauvin]

How did you get it created the first time?

[GinoPane]

@mjauvin when I commented the code, the record was created. I don't fully understand the issue, but for any reason dynamic translatable property got into model's attributes and this exception was thrown: Unexpected type of array when attempting to save attribute "%s", try adding it to the $jsonable property.

This exception was caught in ThemeData::forTheme method and therefore the generic error message was shown on the front end.

Anyway, I committed a fix which should be working fine.