Server list: Check sv_location matches a Gmod flag

[DarthTealc]

Checks that sv_location actually matches one of Gmod's flags. If the sv_location doesn't actually match a flag, it'll ignore the sv_location and continue on to name extraction just like any server that hasn't set a sv_location.

[robotboy655]

What is this meant to achieve?

[DarthTealc]

Allows showing flag from name
Example server: sv_location = "uk", server name = "[UK] Sandbox Server"
In current game:
In PR:

Prevents multiple error flags
Example server: sv_location = "abc", server name has nothing extractable
In current game: Receives a unique "?" flag in the filter list and the server list. Is not combined with other invalid values.
In PR: Receives no flag, same as servers with no location set
Example of current game with 3 unique error flags:

Needed for other PRs
Required for part of #2333, which this PR was split from

Servers without a flag are now filterable the same as any other flag, by being grouped into an "Unknown" country. That flag is always at the end of the flag filter list. This allows you to include or exclude them, the same as all other countries. On the server list itself, they continue to have no flag.

Prevents bypassing this #2367

Servers without a flag are given a higher score. This is to encourage servers to set their sv_location or put it in the server name for automated extraction (eg "[US]"). Either method results in a flag and is therefore not given a higher score.