Infrastructure for caching avatars

Avatars now can be fetched server side and are cached on the server.

Issue #412

Signed-off-by: Michal Čihař <michal@cihar.com>
Standard handling of Avatar settings

Signed-off-by: Michal Čihař <michal@cihar.com>
Migrate get_user_display to new avatar handling

Use server side cached avatars

Issue #412

Signed-off-by: Michal Čihař <michal@cihar.com>
No longer use avatar template tag

We can no use standard URL resolving for that.

Issue #412

Signed-off-by: Michal Čihař <michal@cihar.com>
Remove no longer used avatar template tag

Signed-off-by: Michal Čihař <michal@cihar.com>
Remove no longer used avatar support from utils module

Signed-off-by: Michal Čihař <michal@cihar.com>
Fix import

Signed-off-by: Michal Čihař <michal@cihar.com>
Document new caching of Avatars

Avatars are now downloaded server side and cached for all users. This
improves performace of the site (no need to request to third party
servers) and also improves users privacy for the same reason.

Caching can be configured separately from rest of the Django caching and
it is recommended to use filesystem backed cache for this purpose.

Fixes #412

Signed-off-by: Michal Čihař <michal@cihar.com>
Correct fallback for avatar for None user

Signed-off-by: Michal Čihař <michal@cihar.com>
Use format method instead of % operator

Signed-off-by: Michal Čihař <michal@cihar.com>
Calculate User-Agent string just once

Signed-off-by: Michal Čihař <michal@cihar.com>
Fix typo in docs

Signed-off-by: Michal Čihař <michal@cihar.com>
Document server side caching of avatars

Signed-off-by: Michal Čihař <michal@cihar.com>
Update po files