Move Reading List to Airtable

[ninadicara]

To help us manage the content on the reading list we have decided to try moving it to Airtable. This has a few advantages:

• Multiple categories per item so we can dynamically filter content.
• Form suggestions to streamline adding new content.
• Easier notifications of new suggestions.

This PR:

• Embeds the AirTable form into the Reading List page.
• Adds a page with a suggestion form embedded.
• Will close #146, #158, and #30

Still to do:

• [x] Decide on presentation of the airtable 'view' that is displayed on the website
• [x] Migrate the current reading list content to the new database.
• [x] Add outstanding reading list suggestions from issues
• [x] Document somewhere how the suggestion form review process works (in case we - or someone else - wants to be able to let new people do this in the future). Should maybe include a link to the slack for example and a list of who is an admin there.

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Optional to do:

• [ ] Add to write-up template to update the airtable
• [ ] Allow people to easily download the reading list in some way
• [x] Field on the form for a GitHub username

[ninadicara]

Some initial screenshots of how this would look on the website:

[NatalieZelenka]

Omg @ninadicara this looks so good 😍. I'm obsessed! I can't believe it lets you do form submission as well. I wonder if we would need to be able to have our main table only showing reviewed submissions? I'm sure that would be possible if we'd need to!

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EDIT: Nina told me that this is possible to approve via our Slack due to Slack/Airtable integration. I think maybe we need to document that process somewhere for the future before we pull in this PR, so I'm adding it to a to-do list at the top of this PR.