Copying Timeslider during the build process

[Rub21]

• From: Shift to serving timeslider js and css from main web codebase issues#910 (comment) ,
  @danrademacher, the time-slider files (JS, CSS) and images have been copied during the build process in the web container. Could you take a look to ensure it is working as expected: https://staging.openhistoricalmap.org/?

Other changes in the PR:

• Update comment for start.sh in the web continaer
• Update concurrency and area for seeding vector tiles.

[danrademacher]

The timeslider is working well on staging, so I think this is good to deploy.