Deduplicate GitHub issues in activities.json

[zcorpan]

Fixes #1087

[tantek]

The mozPositionIssue updates all look correct (and finer-grained) to me. Thanks for doing this.

The only nit I noticed (unchanged from before, but worth fixing while touching these since separate issues with descriptions got created) is a couple of instances of:

"mozPositionDetail": "A welcome successor to XBL!",

Could these be replaced with something slightly more specific to the technology being reviewed? (just as the issue numbers are more specific now)

[zcorpan]

What should they say?

[smaug----]

Maybe "Standardized successor of XBL" would be enough?

(Custom elements and Shadow tree aren't really better than XBL, but there is at least good specifications for both.)

[tantek]

Would have preferred text particular to each spec (like what does it provide that replaces what XBL functionality) but better to land this clean up and improve that later.