Cleanups: multiple minor cleanups

ctx, form and model fields, imports order, shorten calls, blank lines,
remove try block, unused imports...
Use elif template tag
Avoid loading unused baseurl template library
Docs cleanups: Move dev definitions to separate section, fix typos
Move 'Report a problem' to a toolbar
Cleanup: removed deprecated fields
Let's rather user cached properties
Followup for previous commit
Cleanup: Generate URL on view
Switch to sane use of url template tag
Avoid using intermediate vars

This way is easier to associate a particular context var with the
reverse call.
Get context in the view rather than in the template

Reduces cruft in the template and provides a better knowledge of the
context that the template is passed.
Remove the baseurl's l filter from all the templates
Remove unused baseurl template filters
Remove unused code on util

After the last changes util.edit only receives links that are callable.
Remove unused baseurl abs_l function
Rely on get_absolute_url() for doing exactly the same thing

This allows to easily remove a baseurl import.
Use reverse instead of l