Dynamically update incident SLA to count down without refresh

Wherever we display an incident SLA, we show the remaining time. This time is currently static and a page refresh is currently required to update it. This causes the display to become out of date for users who leave the incident page open.

To fix this, we could dynamically update the remaining SLA time.

Proposal

• Dynamically count-down the incident SLA timer.
• Update every 15 minutes.
• Avoid requests to the server (since SLA end time currently can't be altered once created)

Out of scope:

• Detecting when the SLA is missed (versus achieved) - separate issue for this here: #270388 (closed)

Original issue: #241663 (closed)

Feature discussion/request: #241663 (comment 405031683)