First version of user extensions

[offtherailz]

Create a minimal version of user extensions plugins (see mockup) to try and refine.

[ElenaGallo]

@offtherailz an observation:
The User Extensions plugin must always be enabled at startup otherwise it is not possible to use the command to enable/disable the other plugins.

How to reproduce

• new context

at the second step:

• add annotation plugin
• enable plugin for user
• add UserExtencions
• enable plugin for user
  
• save
• open the context

Current Result
the user cannot enable the annotations plugin

[tdipisa]

@offtherailz the ** User Extensions ** plugin must be hidden in this step of the wizard and must be automatically included as soon as the administrator has set up an extension for it to appear in the Extensions Library to be activated by the end user.

[francoxyz]

Last comment above are included in the new review list here. Closing