fix(previews): perf & crash issues with thumbnail generation

This commit should immediately fix these issues
by proving a way to limit the number of client-side
previews generated per session, and a pause between
preview generation to avoid pegging the CPU
continuously.

TODO:
- Cleanup templating module code.

- Don’t bother pausing between previews or limiting
them in browsers where we just use CSS to scale
& canvas is not involved.

- Don’t pause before next thumbnail generation
attempt if we’ve already hit the maxCount.
Only increment generatePreviews if an image
without a valid preview has been updated
via server’s thumbnail response.

#1279
Merge branch 'develop' into fix/thumbnail-performance
Merge branch 'develop' into fix/thumbnail-performance
fix(previews): No limits for older browsers.

TODO:
- Cleanup templating module code.

- Don’t pause before next thumbnail generation
attempt if we’ve already hit the maxCount.
Only increment generatePreviews if an image
without a valid preview has been updated
via server’s thumbnail response.

- Update styling doc.

#1279
docs(previews): Document performance-related options.

TODO:
- Cleanup templating module code.

[skip ci]
#1279
chore(travis): allow any *fix branch to be built
chore(travis): remove unused stuff
chore(travis): put back used stuff
fix(previews): cleanup thumbnail module
closes #1279
chore(build): inc build num
#1279