Server convar for loading box customisation

[DarthTealc]

What

Allow servers to specify a convar to set the design of the loading box on the loading screen

Why

Servers often set a loading screen sv_loadingurl with a theme designed after their community. The loading box generally doesn't match this theme.
I previously suggested #2451 but I think it's probably too complex and not worth doing. This new idea is an alternative that I think balances the ability for servers to set a design while preventing servers from hiding the box, and without being too complex to implement.
Why not in the loading screen code? Because it can't be read until the page is loaded. Having it as a sv_ variable would mean it can be applied instantly.

How

Servers would set a sv_loadingtheme to one of a pre-set list of options. This would cause the loading panel to use that pre-set option.
Additionally there would be a cl_loadingthemedefault 1 which when set by the client would force all servers to use the default theme. This would be provided to allow users to opt out.

• sv_loadingtheme is not set, set to something invalid, or the user has set cl_loadingthemedefault 1 - default theme used
• sv_loadingtheme set to one of the valid options - the corresponding theme is used.

Options/Examples

The following options would be available. Rubat could add new options at any time, and users could submit options via pull requests if the option is appropriate to add. Examples are provided as a rough concept and should be improved.
Other metrics such as rounding, borders, etc could be provided in these options but I didn't change those for the examples.

default - the current design. Used by default.

options based around common colours: red, blue, yellow, green, orange, pink, purple, brown, etc. (no examples)
options based on popular gamemodes such as TTT. Would need input from @svdm for this. (no examples)

'white' - a light design

'black' - a dark design

'gmodblue' - based around the Gmod logo colours

tf2 - Team Fortress 2 colours

'hl1' - Half Life 1 colours