Tidy up branches

[afandian]

Remove old branches, merge those that need merging, delete merged and unused ones. Remaining should be master, elastic-2018 and any in-progress branches.

Definition of done:

• All branches merged / deleted.
• Clear where to start when working on a new feature.

[afandian]

Still need to close:

• prep-55-coverage-merge
• title-markup
• condp

They contain unmerged work which we'll need to look at closely.

[ppolischuk]

For the title-markup branch, the agreed-upon spec is available in [this comment] from rest-api-doc/issues/112. (CrossRef/rest-api-doc#112 (comment)). I'm not clear on what work was done in the branch, but at least we have the intended spec.