Replay Index: Add Tooltip describing "Activity" column

[ryan953]

Problem:

Today users don't know the meaning of the "Activity" column on the Replay Index page.

Solution:

Add an information tooltip to the right of "Activity" in the table header on the Replay Index page:

The copy in the tooltip would be "Activity represents how much user activity happened in a replay. It is determined by the number of errors encountered, duration, and UI events."

It should appear similar to other areas of the Sentry app. Here's an example from the Event Details:

[ryan953]

cc @jas-kas

can you help to fill out a description on this please? imo it should say at least:

• where the tooltip appears (either on the column header, do we show it on each row…
• what is the copy