feat(tasks): distinguish template tasks in task list

[sranka]

This PR visually distinguishes tasks that were created from a template in a chronograf task list.

Before this PR
and before influxdata/kapacitor#2621 is merged:

1. A template task is processed on the chronograf server-side to check if it is a rule. It is never in fact.
   1. If a template task is recognized as a chronograf rule, it falsely appears under Alert Rules section.
2. A template task correctly appears in a TICKScripts section. It is however not distinguished from regular tasks that are editable, a template task is not.

After this PR
and after influxdata/kapacitor#2621 is merged:

1. A template task is never processed on the chronograf server-side to check if it is a rule. It cannot appear in Alert Rules section.
2. Tickscript of a template task is visually recognized. It is read-only in chronograf since fix(kapacitor): make templated kapacitor task read-only #5805 .

• CHANGELOG.md updated with a link to the PR (not the Issue)
• Well-formatted commit messages
• Rebased/mergeable
• Tests pass