Use the avatar.HashEmail function instead of hashing email directly.

- **Bug Description**: When assigning User.Avatar the email is not being correctly pre-processed so uppercase characters in emails will cause the wrong gravatar hashes to be used.
- **Gogs Version**:  `Gogs version 0.5.8.1118 Beta`
- **Git Version**: `N/A`
- **System Type**: `N/A`
- **Error Log**: `N/A`
- **Other information**: 
-- I have already fixed this and will submitting a pull request shortly.
-- Originally issue: #700 
-- Original PR (but to wrong branch): #701 

## Steps
- Create/edit a user with the Avatar email:  `erebusbat@gmail.com`  notice that the gravatar loads correctly.  
- Edit the avatar email to be `ErebusBat@gmail.com` and notice the gravatar does not load

According to the [Gravatar API](https://en.gravatar.com/site/implement/hash/) either version should work.