Use gender-neutral pronouns when referring to the player

Currently, Xonotic (inconsistently) uses "he", "him" and "his" to refer to the player, which is not always accurate. This Merge Request changes all instances of these (that I could find with a simple grep) to the appropriate gender neutral version "they", "them", "they're", and "they've". I've done my best to take context into account, changing it only where is relevant to the player (keeping he and him in code comments referencing contributors). This is in an effort to help prevent alienation of players who don't identify as male, and to generally make the game feel a bit more inclusive.

Also I feel like I should mention, after reading the response to #1171 that this MR is not an effort to make Xonotic "woke" or ruin its history as a classic area shooter or in any way dampen its legacy. I simply want this amazing game to be welcoming to more people.