Cleanup: Remove more unused TreeItem methods

Asked Taras about these methods and told that they are unused and that
were added for future potential optimisation that didn't happen.

Followup for 63199a71.
Cleanup: Remove unused view
Bugfix: Display all failing checks in editor dropdown

This is because of 8a0718d0, that is necessary because in the stats
we only need critical checks, but in the editor dropdown we need all
the checks, including cosmetic and functional. This was no problem
for Evernote because they only have critical checks.

This is a dirty hack that needs to be partially undone in a future not
yet created followup commit that detaches get_error_unit_count() from
get_checks(). See the TODO note.
Random cleanups
Recover UI: Allow to filter by goal using dropdown
Recover UI: Only display the goals dropdown if the TP has goals
Recover UI: Send to right URL when blanking dropdown in goal drilldown

Before this when blanking the language or the project dropdown in a
goal drilldown forwarded the browser to broken URLs.
Alter add_project_goals to work with richer phaselist files

This way we can specify the required data in a file and just import it,
instead of having to type each bit of in different forms.

This helps to minimize the current lack of working method to edit goals.
Recover UI: Redirect to TP root if current resource isn't in chosen goal

This avoids showing the current resource when we change the goal in the
dropdown but the current resource doesn't belong to the chosen goal.
Followup: Remove goals statistics table

We now have enough with the next most important goal translate action
and the prioritized goals dropdown.

This also includes some minor cleanups after previous work on
redirecting to TP root if current resource doesn't belong to chosen goal.
Recover UI: Make goal filtered table display all the data
Recover UI: Display actions in a sidebar

The actions sidebar is collapsed by default to make the users adjust to the
fact that we are going to remove this in a future.

If several sidebars are shown then some tabs are displayed to allow users
switch the sidebar.

Extension actions might not work properly.
Recover UI: Display current goal data in sidebar

This includes the ability to edit the goal data in the sidebar.
Recover UI: Display instructions in sidebar

This is a temporary recover until we manage to get this blended into
staticpages.

This includes the ability to edit in the sidebar the instructions for
the current language, translation project or project being displayed.
If no instructions exist it is possible to create them using the same
editing form in the sidebar.

If the overview is for a translation project then the instructions for
the language and the project are also shown (if they have any
instructions), but it is not possible to edit them.

Includes extensive changes to support editing two forms at the same in
the sidebar, because it is now possible to have a form for editing the
instructions and another one for editing a goal.
Recover UI: Automatically display sidebar on extension action output
suggestions: Only increase the count if the suggestion is new
tests: Add suggestion_count tests
tests: Add an accept_suggestion test
caching: Fix post-suggestion behaviour.