Mattermost Support

[pykenny]

Purpose

Recently I'm looking up Mattermost's documentation and feels like it would be useful to add component building support in Mattermost. Given that they offers example for writing apps in Python, plus they're following a lot of Slack's (previous) design/practice, offering extra support for Mattermost message building may be doable and useful.

Details

Mattermost mostly follows legacy practice of Slack:

• Message body only allows plain text, written in Markdown similar to Slack's syntax, compares to Block Kit, which allows app/bot developers to stack multiple different sections in main message.
• For interactive messages, section and interactive components are all placed under attachment part of the message, as Slack's legacy approach did (Interactive Message Documentation)
• Structure of interactive dialog seems very similar to legacy slack dialog as well. (Form Documentation)
• Mattermost only implements a subset of form/interactive components compared to Slack. And seems that they're not adding more of them in the short term.
  • For instance, date/time/datetime picker: Implement datetime picker in Interactive Dialogs mattermost/mattermost#15413

By doing so, we can also consider reviving #43 to allow building attachments in Slack messages until Slack fully deprecates support.