Transition Vue app to read (and write?) markdown fields from graphql API

[ngottlieb]

Purpose is to migrate fully to the markdown API so we can be fully consistent with the React Native app.

Read markdown fields:

• posts.detail_md
• reaches.description_md, reaches.gaugeinfo_md, reaches.shuttledetails_md
• accidents.description_md
• rapids.description_md
• posts_photos.description_md (TBD)

Writing markdown fields — applies to same fields, but remains an open question, particularly surrounding what we want the edit interface to look and feel like. We likely don't want users having to actually type in Markdown as one would with a standard Markdown editor, so need a rich text editor that outputs markdown.

[ngottlieb]

Some things worth noting as I work on this:

• while alerts can hold HTML via the data model, in practice, they are generated using a plain text field, not a rich editor. If/when these get remodeled away from the overloaded posts table, we can do away with the markdown caveat here
• reaches.gaugeinfo and reaches.shuttledetails are not currently accessible in the Vue app
• accidents.description_md is not used by the Vue app as we just display a table and link to existing Laravel (and, soon, Wordpress) details

[ngottlieb]

After a discussion on Slack with @tallowen, decided to move forward with the "read Markdown fields" portion of this task first, and then to coordinate with @drewalth on a rich Markdown text editor as he gets closer to implementing that on the frontend.