Add support for custom avatar provider like Libravatar or self-hosted service

[Vinrobot]

This PR fixes JavaScript build process with the new NPM dependencies and adds the possibility to use a different user avatar service like Libravatar.

[ssddanbrown]

Thanks for the pull request @Vinrobot.

After merging, I think I'll go through the existing wording and functions in the code and essentially 'de-gravatar' all areas to make them generic 'avatar' wording instead but with gravatar still as a default. Just warning now as I'll probably reduce and re-name some of the env variable names.

Just one thing, Could you remove and changes that relate to the build process? Will make this request neater and I've just gone through and applied a more longer-term fix by updating all modules to standard ES6 import/export syntax which has caused conflicts with this. I know it's 100% my bad though for leaving the build process broken. Feel free to open on a new pull request if needed.

Since this a a new feature and since I might break an old .env var this will be targeted for the next feature release rather than being in the next minor release.

[Vinrobot]

Hello @ssddanbrown,

I removed the two commits related to the build process.

[ssddanbrown]

Now merged, ready for the next release.

Just so you're aware after updating, I have changed some of the config for this:

• GRAVATAR_URL will now be AVATAR_URL
• Placeholders will now use a $ instead of a % in the format, So: ${hash} ${size} ${email}. This is just to align it with some other BookStack options that have placeholders.

Details of changes are in b56fc21.
Thanks again @Vinrobot!