Overhaul /user/tasks page.
Overhaul /user/tasks page.

- Fix update_filter() so it stops losing the tags.
- Fix 'Next page' link to properly encode query parameters.
- Move 'Next page' link higher up. It was too low, people weren't seeing it.

R=stip@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1477883002

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py