Bind providers to a model state

[bdmendes]

Closes #1056
Closes #726

This PR greatly restricts the Provider usage and makes its usage more compact, without the RequestDependantWidgetBuilder. It also makes accesses to SharedPreferences synchronous.
LazyConsumer now forces the handling of an empty state, which forces a refactor of several widgets.

Review checklist

• Terms and conditions reflect the current change
• Contains enough appropriate tests
• If aimed at production, writes a new summary in whatsnew/whatsnew-pt-PT
• Properly adds an entry in changelog.md with the change
• If PR includes UI updates/additions, its description has screenshots
• Behavior is as expected
• Clean, well-structured code

[bdmendes]

@LuisDuarte1 @thePeras I included several TODOs with the refactors yet to do. Can you help with them? I'll be quite busy for the next two/three weeks, and this is a bit unstable now.

[thePeras]

There is only left to refactor the lectures page consumer

[codecov]

Codecov Report

Merging #1057 (cd0eb29) into develop (8c09035) will increase coverage by 1%.
Report is 1 commits behind head on develop.
The diff coverage is 21%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop   #1057    +/-   ##
========================================
+ Coverage       16%     17%    +1%     
========================================
  Files          207     206     -1     
  Lines         6483    6381   -102     
========================================
+ Hits          1004    1032    +28     
+ Misses        5479    5349   -130     

[thePeras]

Lectures page lost its refresh feature when it's empty, but that will be cover by #909.

Nice work deleting more than 200 lines in this refactor!

[LuisDuarte1]

LGTM 🚀