Allow to disable SSL verification for Velruse

Python2 stdlib SSL module doesn't support SNI [0], so we allow to
disable the SSL verification.

[0] https://github.com/kennethreitz/requests/issues/749
Fix c.instance check

Somehow c.instance can be an empty string (this is the default for
pylons template_context objects).
Show short category description in category description view

This view is ment to be embedded. But the height of the long description
can vary in height because it can contain e.g. images. So instead we use
the short description which has a limit of 255 chars and no
HTML/markdown.
Fix AuthCheck.propose_login

We now propose login if all permission refusals are due to not being
logged in or not being a member of this instance.

This is relevant due to the more restrictive membership checks in
5205fa1db0a11b42b0a3db80cb5ad5ed6917577d and
4cfefbe2920f276357462da424a8d7baa038d0e7.

Practically this means that by default the "new proposal" button / link
is now displayed again for non-logged-in users, and that if they visit
`/proposal/new` (that link), they're now redirected to the login page
instead of getting a 403.
Merge pull request #934 from liqd/feature-short-category-description

Show short category description in category description view
Merge pull request #935 from nidico/feature-fix-authcheck-propose-login

Fix AuthCheck.propose_login
Fix display of user role in listing

This error caused user badge triggered user group assignements from one
instance to apply on others.
Merge pull request #936 from nidico/feature-fix-user-row-role-display

Fix display of user role in listing
update supervisor (3.0 -> 3.1)
Fix config.get_value allow_overwrite for non-instance case (fixes #937)

Fixup e238fe5a, introduced in #915.
Disable broken mako debugging

As pylons provided mako debugging has been broken for some time and
makes things even worse, this commit disables it entirely.

The broken custom handle_mako_error resulted in error tracebacks
stopping at the place where mako error handling would normally take
place, instead of getting down to the actual error. This is especially
relevant if the error didn't occur in the template, but in other code,
which was called from within a template.
Merge pull request #939 from nidico/feature-disable-broken-mako-debugging

Disable broken mako debugging
Merge pull request #938 from nidico/feature-937-fix-allow-overwrite

Fix config.get_value allow_overwrite for non-instance case (fixes #937)
Add CHANGES for 2.5.1
Merge tag '2.5.1' into feature-geo