files: added branch permissions checks into web edit operations.

branch-rules forbid removal of permissions assigned.

permissions: add links to permissions summary for each permission pages

branch permissions: added logic to define in UI branch permissions. - db models - new permissions pages placeholders

error-tracker: use nicer display for exception generated by rhodecode.

ssh: improve logging, and make the UI show last accessed date for key.

repo: re-order creation/fork forms for better UX and consistency.

select2: use wider search field, better UI.

caches: fix import of compute_key_from_params function.

caches: don't try to delete keys if there aren't any to delete. - This is problematic for REDIS backend.