Add a @medic/constants shared library

[dianabarsan]

Describe the issue
We have a bunch of magic strings spread throughout the app.
One of the prominent ones being mm-online: https://github.com/search?q=repo%3Amedic%2Fcht-core+mm-online&type=code
This is not the only example.

Describe the improvement you'd like
Collect all reused magic strings and add them to a shared library.

[garethbowen]

Examples I can think of...

• mm-online
• X-Request-Id
• _local/transitions-seq
• _local/background-seq
• doc types, eg: translations
• doc IDs, eg: service-worker-meta

[dianabarsan]

Thanks for adding to the list. I agree with all these suggestions.
We also have X-Medic-Service header that we pass to haproxy in both api and sentinel.
For doc ids, we can also make it return translation doc names like (code) => messages-${code}.
Same applies to form names, other docs that have prefixes (org.couchdb.user:).

I think we're going to end up with a sizeable list.