Validate dinkurs key with Dinkurs API

[patmbolger]

PT Story: https://www.pivotaltracker.com/story/show/158426458

Validation of the dinkurs_company_key attribute is complicated by these factors:
• We need to invoke the "fetch" API in order to determine if we have a valid key (that is, there is not a separate endpoint we could use to simply check key validity without fetching events)
• We want to avoid calling that API end point more than once, since it is called synchronously (which is the only reasonable way to do it)
• To fetch and store company events, we need a company ID
• Therefore, we need to fetch events after the company has been saved (for create)

Changes proposed in this pull request:

1. Company create:

• After other attributes are saved - if successful - try to fetch events (if key has been specified)
• If fetch fails (due to invalid key), set key validation error and tell user that the company was successfully created but that there was a problem with the key

2. Company update:

• If all other changed attributes are valid, try to fetch events (if key is specified)
• Show error(s) to user if any attributes are invalid or dinkurs key is invalid

3. Miscellaneous small changes

Screenshots (Optional):

Company created successful, but dinkurs key is invalid

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Company updated with invalid dinkurs key

Ready for review:
@AgileVentures/shf-project-team

[houndci-bot]

Lint/AmbiguousRegexpLiteral: Ambiguous regexp literal. Parenthesize the method arguments if it's surely a regexp literal, or add a whitespace to the right of the / if it should be a division.