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)

- Allow to delete current avatar

The "PHP" formatting function doesn't add anything, except an undocumented date format.

All usages in the templates have been replaced with DateFmtShort and DateFmtLong for convenience.

Correction for #723
Bug was : decode failed if the password contains ```:```

Related to #700

In the original bug report it was referencing only the sytem avatar images for setup users (like in the header); however the problem also persists with things like commit history.

This commit fixes the `tool.AvatarLink` function so that it also uses the already existing `avatar.HashEmail` function.

I also refactored the `tool.AvatarLink` method some to make the control flow more apparent and adhere better to DRY (there were multiple calls to the `EncodeMd5` function that the `HashEmail` function call replaced, now there is only one.)