Add IMAP email provider

[ellienieuwdorp]

Content

• Add an IMAP-based email provider
• Updated and redesigned settings UI to allow for IMAP configuration
• Refactored some existing gmail code to be in line with the new provider approach

TODO

• Update docs with setup guidance
• IMAP folder fetching/auto-population in the UI (lots of email providers have unintuitive folder names)

Nice to haves

• IMAP host/port presets in the config UI for common email providers
• Secure keychain credential storage
• Some kind of automated testing setup for different providers

Structure
The root folder is getting a bit crowded now, maybe a folder structure could be introduced? Could also be in a follow-up PR.

.
├── bcfeed.py
├── server.py
├── paths.py
├── pipeline.py
├── dashboard/
│   ├── dashboard.html
│   ├── dashboard.css
│   └── dashboard.js
├── mail/
│   ├── email_provider.py
│   ├── provider_factory.py
│   ├── gmail_client.py
│   ├── gmail_provider.py
│   ├── imap_client.py
│   └── imap_provider.py
├── bandcamp/
│   ├── email_parser.py
│   └── page_parser.py
└── docs/
    ├── SETUP.md
    ├── GMAIL_SETUP.md
    └── ...