* Add support for federated avatars

Fixes #3105

Removes avatar fetching duplication code
Adds an "Enable Federated Avatar" checkbox in user settings
(defaults to unchecked)

Moves avatar settings all in the same form, making
local and remote avatars mutually exclusive

Renames UploadAvatarForm to AvatarForm
as it's not anymore only for uploading

* Run gofmt on all modified files

* Move Avatar form in its own page

* Add go-libravatar dependency to vendor/ dir

Hopefully helps with accepting the contribution.
See also #3214

* Revert "Add go-libravatar dependency to vendor/ dir"

This reverts commit a8cb93ae64.

* Make federated avatar setting a global configuration

Removes the per-user setting

* Move avatar handling back to base tool, disable federated avatar in offline mode

* Format, handle error

* Properly set fallback host

* Use unsupported github.com mirror for importing go-libravatar

* Remove comment showing life exists outside of github.com

... pity, but contribution would not be accepted otherwise

* Use Combo for Get and Post methods over /avatar

* FEDERATED_AVATAR -> ENABLE_FEDERATED_AVATAR

* Fix persistance of federated avatar lookup checkbox at install time

* Federated Avatars -> Enable Federated Avatars

* Use len(string) == 0 instead of string == ""

* Move import line where it belong

See
https://github.com/Unknwon/go-code-convention/blob/master/en-US/import_packages.md

Pity the import url is still the unofficial one, but oh well...

* Save a line (and waste much more expensive time)

* Remove redundant parens

* Remove an empty line

* Remove empty lines

* Reorder lines to make diff smaller

* Remove another newline

Unknwon review got me start a fight against newlines

* Move DISABLE_GRAVATAR and ENABLE_FEDERATED_AVATAR after OFFLINE_MODE

On re-reading the diff I figured what Unknwon meant here:
https://github.com/gogits/gogs/pull/3320/files#r73741106

* Remove newlines that weren't there before my intervention

Use URL query parameter render=1 to render content in raw mode.

- Fix go-gogs-client#10
- Related to #809

No guarantee for compatibility with future changes

Both are possible on explore and admin panel

Collaborators have write access as default, and can be changed via repository
collaboration settings page to change between read, write and admin.

- Allow to delete current avatar

- Migrate: #2704 #2653
- Clone: #2701
- Mirror, Pull

Improve test cases, config settings, also show SSH config settings on admin config panel.

Remove the possibility of using email as user name when user actually push
through combination of email and password with HTTP.

Also refactor update action function to replcae tons of arguments with
single PushUpdateOptions struct.
And define the user who pushes code as pusher, therefore variable names shouldn't
be confusing any more.