Update to the SuperTuxKart source strings

[Alayan]

The source strings for the game's translation have been updated on Transifex. Languages that were previously 100% translated are now 88,87% translated.

Changes include:
- Translation for the STK server messages used during log in
- Translation for the names of STK's default skins
- Translation for the server answers to server commands in the server lobby (and much improved base messages in english)
- Contextual translation for several words that needed it (most notably "Medium" which could be a kart class, a font size, a graphics setting...)
- Removal of raw URLs from translatable strings
- Some other new strings and various small string fixes

Information about how to help translating STK is available here. 

We had some cases previously of low-quality translations that ended up wasting our time, so please only contribute if you have a solid grasp of English and your chosen language. 

If you notice issues in the source strings that need to be translated, please let me know!

[Tuxito3000]

I'd like to help with the Spanish translation (it's my language), but it's asking for a business email address. How do I do that? :p

[Alayan]

I notice I didn't answer earlier. I suppose you figured it out, but you should be able to use any email address to sign up to Transifex to become a translator. However I think in some contexts the email address may be shown (team managers can see them, not sure if it's anywhere in the translation credits).

In general, it's good practice to have some separation between email addresses depending on the main use cases (don't use the same email address for your banking stuff and for gaming stuff for example), it helps to reduce tracking and reduce the attack surface for malicious actors.