Restore "Sticky Issues Header" feature

[fregante]

It was added in #1311 but I can't find it anymore.

Anyone remember if it was removed for any reason? cc @tommoor @hkdobrev @yakov116

Anyway this is a quick fix if anyone wants to pick it up.

[sharkykh]

$ git log -S'/* Sticky table header on issues list */' --pretty=format:'%h %an -- %s -- %ad'
a9ec680 Waseem Dahman -- Fix sticky file header tooltips (#1581) -- 2018-11-06 17:43:48 +0200
dd8760b Tom Moor -- 📌Sticky Issues Header (#1311) -- 2018-05-10 20:13:57 +0300

a9ec680 combined this and the sticky file headers features together, and changed the comment from /* Sticky table header on issues list */ to /* Sticky file and table headers */

$ git log -S'/* Sticky file and table headers */' --pretty=format:'%h %an -- %s -- %ad'
76cc04d Federico Brigante -- Drop feature `make-headers-sticky` -- 2019-03-18 19:18:55 +0200
a9ec680 Waseem Dahman -- Fix sticky file header tooltips (#1581) -- 2018-11-06 17:43:48 +0200

76cc04d removed both sticky issue list headers and sticky file headers (the latter was implemented by GitHub).

To whoever picks this up,
keep in mind that dd8760b (#1311) also had a bugfix commit later: 4f43ef6 (#1318)
And maybe it should have an option to disable - so, have a .tsx file to inject a rgb-sticky-header class, like hide-watch-and-fork-count does.

[fregante]

Excellent! Thank you so much for that!