* Add MaxDisplayFileSize setting

* Don't show files that are too large

* Localized FileTooLarge

* Change IsFileTooBig => IsFileTooLarge

Re: issues gogits/gogs#2863 and gogits/gogs#3231

As a result of modifications to the contents of the conf folder, `make bindata`
was run, causing an update to bindata.go.

Meta
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This commit will be rebased onto the 'develop' branch.

Initially proposed by #2376 and fixes #3068 as well.

- Update locale bindata

- Update language choose dropdown to fit more languages

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

- Remove unused config settings `[picture] service`

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

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

Config option [server] SSH_LISTEN_PORT to the port the builtin SSH server will be listen.
It can be different from SSH_PORT which is supposed to be exposed in the clone URL.
This should solve the problem when user runs Gogs inside Docker container
and still want to use builtin SSH server.