[added] Thumbnail links support

[mdziekon]

This PR implements links support for <Thumbnail> component as <ThumbnailLink>.

This is how visual test looks like:

(note: as you can see, I've used Holder.js generated placeholder for the thumbnail, but I'm not sure whether "/assets" dir is a good place for it or not. If anyone have a better idea, please let me know.)

[AlexKVal]

assets folder is OK (R-B does the same)

LGTM ❇️

Though I'm not sure about:
This PR changes the minimum version of R-B dependency:
from 0.15 to

"react-bootstrap": ">=0.22.4"

here

Anyway it needs one more approval of @react-bootstrap/collaborators

If this PR merged as is, then the next version of R-R-B will be 0.19 because of this "react-bootstrap": ">=0.22.4" change, I presume.

[mdziekon]

According to R-B's Changelog, Thumbnail component was introduced in version 0.22.4 (https://github.com/react-bootstrap/react-bootstrap/blob/master/CHANGELOG.md#v0224---mon-18-may-2015-165306-gmt)

I guessed dep version had to be changed to make sure we have this component in library.

[AlexKVal]

Ah.. Did think about it, sorry 😊

[dozoisch]

LGTM

[mtscout6]

LGTM

[taion]

I wonder if we can use some wrapper component for these generic linked components - i.e. just pass down getLinkProps to the wrapped component.