Use "Enable Gravatar" but not "Disable" (#36771)

* Fix #35685
* Fix #35627
* Fix #31112


Introduce "fipped" config value type, remove unused setting variables.
Make DisableGravatar=true by defult, remove useless config options from
the "Install" page.

The legacy config options are still kept because they are still the
fallback values for the system config options.

---------

Signed-off-by: wxiaoguang <wxiaoguang@gmail.com>

tests/sqlite.ini.tmpl

@@ -37,7 +37,6 @@ SSH_LISTEN_HOST  = localhost
 SSH_PORT         = 2203
 START_SSH_SERVER = true
 LFS_START_SERVER = true
-OFFLINE_MODE     = false
 LFS_JWT_SECRET   = Tv_MjmZuHqpIY6GFl12ebgkRAMt4RlWt0v4EHKSXO0w
 ENABLE_GZIP      = true
 BUILTIN_SSH_SERVER_USER = git
@@ -59,10 +58,6 @@ DEFAULT_KEEP_EMAIL_PRIVATE        = false
 DEFAULT_ALLOW_CREATE_ORGANIZATION = true
 NO_REPLY_ADDRESS                  = noreply.example.org
 
-[picture]
-DISABLE_GRAVATAR              = false
-ENABLE_FEDERATED_AVATAR       = false
-
 [session]
 PROVIDER = file