Updated and refactored message rendering

[benwarfield-usds]

Work that was originally undertaken to update upload-preview and link-preview rendering, which expanded to handle more complicated bot-generated messages (from e.g. the Jenkins or Github bots), and culminated in some significant refactoring, in order to make it simpler to fix various rendering issues that were found in that process.

Commits in the branch are separated out as cleanly as seemed possible, to give some traceability, but at a high level:

• moved as much HTML generation as possible from the python code to the template
• updated regular-expression-based parsing to have fewer special cases and to handle the IDs as currently generated by slack (which don't all match the older regex)
• handle most special-case slack markup directly, rather than translating it to markdown (there are unquestionably some cases that are handled badly by this, which are different from the cases that were handled badly before: markup engines are hard to reverse-engineer)
• handle more cases of the "attachments" list than were previously handled

[benwarfield-usds]

I tested with all the cases that I have available—one of the frustrations I had was that Slack appears to have changed their format and not to make it especially easy to figure out what changes have been made. So it's possible that I broke something that I wasn't able to test, but I did test this on several days worth of slack messages in several organizations with a few different bots attached.

It would be interesting to try to set up a workspace that has a a reasonable number (more than 100 and less than 10K) of messages in it, and just periodically re-export it to see what's changed and make sure everything still works—I'm not sure that's within my available capacity for this particular side-line, however.