Enable/disable input based on user chosen options for wiki and issue tracker.

Sets all avatars to use PNG image format.
Keeps avatars consistent at 290x290px resolution.

Signed-off-by: Dennis Chen <barracks510@gmail.com>

RepoLink -> Link, RepoRelLink -> RelLink, FullRepoLink -> FullLink

* Add MaxDisplayFileSize setting

* Don't show files that are too large

* Localized FileTooLarge

* Change IsFileTooBig => IsFileTooLarge

Executed go fmt

getEngine() not handles DB parameters (#2972) (#2974)

Uses .AllCols() for Update in updateMirror()

Spanish traslation removed

Fixed a wrong way to ommit the --prune option in process.ExecDir() for MirrorUpdate function

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

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

Also changes the avatar from a jpeg to a png, to allow for
transparent background. The indexed png is also smaller in size.

Note that at the moment the default avatar is only used when
the user requested a custom avatar and the custom avatar file
is not found (should never happen).

In the future the default avatar could be used as a default
return when by-mail avatar lookups fail too (both gravatar
and libravatar support passing a default)

- Update locale bindata

From https://godoc.org/crypto/tls#Config

    ServerName is used to verify the hostname on the returned
    certificates unless InsecureSkipVerify is given. It is also included
    in the client's handshake to support virtual hosting unless it is
    an IP address.

This is needed for certificate validation without InsecureSkipVerify.

* Do not write HTML in text/plain mail part

Fixes #2928

* Pass text/plain first, text/html second

* Do not send plain/text email if html2text failed (untested)