Switch to chart.js for the release activity page

[pietroalbini]

This PR switches the charts in the /releases/activity page from Highcharts to chart.js, improving multiple things:

• Highcharts is proprietary, and I'm not actually sure docs.rs fits into the cases where we don't need a commercial license for it.
• Highcharts doesn't fit well with CSP, requiring style-src 'unsafe-inline' to work.
• This removes the last dependency we fetch from cdnjs, so this fixes self-host all static content #921.

Rendered example of the page with chart.js:

[Nemo157]

Code LGTM. I haven't tested it locally and I assume we have no tests covering this page, but I trust your screenshot.

[Nemo157]

Test failure is probably #1251 making the test flaky, probably needs reworking to inject more test crates to ensure there is a result.

[pietroalbini]

If anyone wants to test it locally I recommend running this query to fill some data in:

UPDATE config SET value = '{"counts":[233,281,431,564,390,188,230,291,320,352,458,332,349,338,358,410,467,579,841,397,565,298,407,532,511,456,484,426,326,295],"dates":["19 Dec","20 Dec","21 Dec","22 Dec","23 Dec","24 Dec","25 Dec","26 Dec","27 Dec","28 Dec","29 Dec","30 Dec","31 Dec","01 Jan","02 Jan","03 Jan","04 Jan","05 Jan","06 Jan","07 Jan","08 Jan","09 Jan","10 Jan","11 Jan","12 Jan","13 Jan","14 Jan","15 Jan","16 Jan","17 Jan"],"failures":[16,9,6,7,29,8,6,8,4,13,18,12,11,4,19,18,28,44,23,17,14,8,10,28,19,14,7,13,8,17]}' WHERE name = 'release_activity';

I assume we have no tests covering this page

Yeah, we don't really have a way to do UI tests as of today.

[jyn514]

LGTM but I would like to deploy before merging this, there are quite a few PRs queued up to be deployed.