Make origin selector multiple choice

There are some trees which are being tested on more than one origin within the same tree name, branch and checkout.

Update the frontend selector so that you can select multiple origins
Update the frontend search parameters so that it is an array of strings
Update the backend to use an IN operator if the origin array is bigger than 1 in length, and not have any origin filter if it is empty

Start by locating the frontend origin selector and the code that builds frontend search parameters, then trace the corresponding backend filtering logic. Update the selector and parameters to support multiple origins, and verify that empty, single-origin, and multi-origin searches apply the specified filtering behavior.

Written by the indexing model from the issue text.