Add avatar server URL configuration support

[siberianlove]

Sometimes it's useful to specify a different avatar server, like in my case, where our company has its own Gravatar-compatible service with employee avatars.

Gravatar uses an email hash to map avatars, but companies quite often have business mail with their own domains, and they have some sort of requirements for employee IDs and avatars on corporate portals and services.

[jcormier]

Were you able to verify the default gravatar support still works? I'm not sure how to test it.

[siberianlove]

Yes it works, i tested it. You can test it with administrator rights by going to Administration > Settings > Display or using url https://your-redmine/settings?tab=display
Redmine displays current used gravatar server. And by default it will be https://gravatar.com
Also you can enable "Use Gravatar user icons" checkbox and visit any user page. Users will have avatars with this checkbox enabled. And if you specify REDMINE_AVATAR_SERVER_URL value of this env will be displayed at Administration > Settings > Display page and redmine will use this gravatar server for displaying avatars.

default one:

https://seccdn.libravatar.org specified in REDMINE_AVATAR_SERVER_URL:

[jcormier]

Much appreciated

[jcormier]

Do you want this cherry-picked to any previous releases?

[jcormier]

Released in 5.1.2

[siberianlove]

Do you want this cherry-picked to any previous releases?

Released in 5.1.2

5.1.2 is enough for me, thanks a lot!