Add 1pMobile GB (MMS and internet)

Fixes #20

I mostly just copied the existing Asda Mobile entry (I added it before the 3 entry, since 1 < 3 🤷). The file passes xmllint.

I've copied the XML file to /usr/share/mobile-broadband-provider-info/serviceproviders.xml and the Access Point Names appear in gnome-control-center and mmsd-tng no longer complains.

Question: Two separate entries appear, both called 1pMobile with APN data.uk. Should these two apn entities be merged into one (containing two usage elements, one with type="internet" and the other with type="mms")?

Note: The MCC/MNC codes match an existing entry for EE, which I assume means 1pMobile is an MVNO piggybacking on their network. I set primary="false" on the provider element, since that seems to correspond to other MVNO entries; but I'm not totally sure this is correct (this is my first time playing with these settings). Would appreciate a check by somebody who understands this more; I really don't want to accidentally override anyone's existing EE settings!