Do not assume avatar needs be changed from gravatar.com (#3209)

Always send user to settings screen to change avatar.
Drops "change_custom_avatar" localized message, keeps "change_avatar"
for the generic one.

NOTE: only changes the en-US locale, as per
https://github.com/gogits/gogs/wiki/Contributing-Code#those-we-do-not-accept

NOTE: requires rebuild of bindata.go to fully see effects

conf/locale/locale_en-US.ini

@@ -220,8 +220,7 @@ org_still_own_repo = This organization still has ownership of repositories, you
 target_branch_not_exist = Target branch does not exist.
 
 [user]
-change_avatar = Change your avatar at gravatar.com
-change_custom_avatar = Change your avatar in settings
+change_avatar = Change your avatar
 join_on = Joined on
 repositories = Repositories
 activity = Public Activity