feat(accessibility): Improve search input fields accessibility

[mgcm]

This PR improves the accessibility of a couple of search fields:

• adds an accessibility label to the Search Participants input text field in the Participants Pane
• adds an accessibility label to the Search input text field in the Participants Stats dialog with added context and the associated translations

[jitsi-jenkins]

Hi, thanks for your contribution!
If you haven't already done so, could you please make sure you sign our CLA (https://jitsi.org/icla for individuals and https://jitsi.org/ccla for corporations)? We would unfortunately be unable to merge your patch unless we have that piece :(.

[mgcm]

@jallamsetty1 or @Calinteodor would it be possible to get a review on this? thanks!

[Calinteodor]

Please jenkins test this please!

[mgcm]

@Calinteodor seems like all checks have passed :)