[A11y][Screenreader] Provide form and tab selection updates to the user

[Dananji]

Description

When using the timeliner with a screen-reader the following issues were identified related to the IIIF Manifest selection modal on the home page,

Screen reader/keyboard a11y observation	Impact and/or possible solutions
When a tab is selected for either typing in a Manifest URL or for uploading a .json file the screen reader doesn't announce selected tab. ❌	User doesn't get any feedback upon the tab selection, this change is only visual.
When focus is on the Manifest URL form input, the screen reader don't see a meaningful label associated with the input field ❌	The input field must be associated with the visual text label, possibly using aria-* attributes.

Part of avalonmediasystem/avalon#6435.

Done Looks Like

• Provide tab selection updates via screen-reader
• Add screen-reader accessible text label to the input field