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.)

According to the [Gravatar API](https://en.gravatar.com/site/implement/hash/) whitespace should also be removed from the email, it was not doing this previously.

Not enough arguments in call to c.AddFunc when I copied and pasted