annotation fails for tweet with camelcased username

Sample tweet: https://twitter.com/realDonaldTrump/status/979326715272065024

**Annotation**: https://hypothes.is/api/search?uri=https://twitter.com/realDonaldTrump/status/979326715272065024

**Expected result**: Annotation appears in H client at the tweet URL

**What happens**: It doesn't

**Why not**: Twitter declares:

``

so the client's search ends up being

`https://hypothes.is/api/search?uri=https://twitter.com/realdonaldtrump/status/979326715272065024`

which fails.

**Possible solution**: When the search URI obtained from the target page is a case-insensitive match for the `rel="canonical"`, include both in the set of URIs the client searches for.

Here's where such an intervention might happen:

https://github.com/hypothesis/client/blob/2578fefde9011a907411e1b75bad93d04b539445/src/sidebar/store/modules/frames.js#L74

Both variants are available in that context: