EPIC: As a new user I want to join HfLA Slack so I can communicate with project teams

### Overview
As a volunteer getting onboarded, I need to join the HfLA Slack workspace as a part of it. The onboarding workflow should automatically invite me to Slack, verify that I have accepted the invitation, and guide me through the process if I have not yet joined.

This feature replaces the current manual process where users are directed to external Slack links. Instead, VRMS will manage Slack onboarding directly within the onboarding flow.

### Action Items
- [ ] Sends Signupemail per user a Slack invite; triggered by clicking “save/continue” on email collection screen (3.06)
- [ ] Clicking “confirm” uses Slack API to confirm if email address exists in HfLA Slack workspace
- [ ] Successful API check (i.e. “yes, Slack user exists”) saves user’s Slack ID to user and proceeds to GitHub screen (3.08)
- [ ] Unsuccessful API check (“no, Slack user doesn’t exist”) displays failure text
- [ ] Clicking “resend” manually resends Slack invite
- [ ] Email for help is a “mailto:” link

### Resources/Instructions
[Main design (final)](https://www.figma.com/file/9c6pxjImN8MjHAZLH7zxYm/VRMS-Mocks?node-id=25469%3A3596)
[Fail text (final)](https://www.figma.com/file/9c6pxjImN8MjHAZLH7zxYm/VRMS-Mocks?node-id=25469%3A3642)