New check: Metadata has valid date

[simoncozens]

What needs to be checked?

If date_added is empty or malformed, things Get Bad. We should check that. Probably not as a self-contained check, but as part of something else that is checking METADATA.pb

Suggested profile

Suggest which profile the check should be added to. The most common are:

• [X] Vendor-specific: Google Fonts
• [ ] Vendor-specific: Adobe Fonts
• [ ] OpenType (requirements imposed by the OpenType specification)
• [ ] Universal (broadly accepted best practices on the type design community)
• [ ] Other:

Suggested result

Which log result level should the check have:

• [X] 🔥 FAIL (An issue that must be corrected for the font to function properly)
• [ ] ⚠️ WARN (A potential issues that may need to be addressed)

Severity assessment

5, causes servers to fail.

[felipesanches]

This kind of problem happened recently in this case: https://github.com/google/fonts/pull/7694/files

[felipesanches]

I'm assuming here that the only acceptable format is YYYY-MM-DD

[felipesanches]

Even though you suggested adding this validation to some preexisting check, I opted here to create a new check because I think we should address separately and more carefully that matter of organization of (and an eventual clumping of) checks.

[vv-monsalve]

This had broken the push to dev, so the status for this should be ☠️ FATAL

[felipesanches]

Yes, @simoncozens had proposed FAIL but I actually made it FATAL.

[felipesanches]

https://github.com/fonttools/fontbakery/blob/687e6fa1948910b5ba4cc612e8efc4f43faf0e5d/Lib/fontbakery/checks/googlefonts/metadata.py#L84-L85

https://github.com/fonttools/fontbakery/blob/687e6fa1948910b5ba4cc612e8efc4f43faf0e5d/Lib/fontbakery/checks/googlefonts/metadata.py#L102-L106