Allow translating “Join Game” differently

[Adrián Chaves]

In the main menu, *Join Game* stands for joining a game that you still have to specify. In the lobby, it stands for joining the game currently selected in the list of games.

Some langauges (e.g. Galician) require to translate “Join Game” differently in each of those contexts.

Note: In addition to providing an i18n context to allow different translations, provide a translator comment to explain what each “Join Game” stands for.

[Itms]

The strings are already separated, and can be translated separately, thanks to the POT splitting.

However, I add translation comments as requested, to ease localization.

[Itms]

In 15775:

Add a translation comment for "Join Game" buttons in the main menu and the lobby, to clarify the translation context.

Fixes #2548