Added and updated Spanish translations

Updated the following files:
1. `client.es.yml`
2. `server.es.yml`
3. `server.en.yml`

Our goal was to remove the most glaring omissions from the Spanish versions of the .yml files, where "glaring" defined as missing yaml errors showing up in the most regularly-traversed parts of the platform.

We focused on the admin panel(s) and main user pages. 

We added a lot of missing yaml (relative to the latest version of their English counterparts) to both `client.es.yml` and `server.es.yml`.

We also found a handful of English typos/grammatical errors, hence the edits to `server.en.yml`. 

We hope you find these changes to be improvements and amenable!

cc @jsl