Add private leagues back into trader

[vaisest]

Fixes #10211

Description of the problem being solved:

Private leagues were removed from the trader with the removal of POESESSID. However, we do have the account:leagues scope, which means we can use the matching OAuth endpoint to fetch the leagues.

This also fixes an issue where the "Search for" button only ever opens the PC site, since I noticed that just now. (Thankfully nobody plays on console).

Steps taken to verify a working solution:

• todo

Sadly I'm not actually in any private leagues, so while it does clearly fetch the leagues

Link to a build that showcases this PR:

Before screenshot:

After screenshot:

[everix1992]

For what it's worth, I pulled your branch down and did a test run and was able to see my private league