[WIP] - UI changes to show list of Custom Button Events

[h-kataria]

Displays link to Custom Button Events list from supported CI summary screens. Currently this link is only visisble to super admin and admin roles. Custom Button Events list renders gtl, links on the list are not clickable.

Fixes https://bugzilla.redhat.com/show_bug.cgi?id=1511126

[skateman]

@h-kataria were we both working on this 😮 #4514

[miq-bot]

Checked commit h-kataria@769c5a6 with ruby 2.3.3, rubocop 0.52.1, haml-lint 0.20.0, and yamllint 1.10.0
7 files checked, 2 offenses detected

app/controllers/mixins/explorer_show.rb

• ❗ - Line 228, Col 7 - Style/RedundantConditional - This conditional expression can just be replaced by !(db == CustomButtonEvent).

app/helpers/textual_mixins/textual_custom_button_events.rb

• ❗ - Line 8, Col 8 - Style/NumericPredicate - Use num.positive? instead of num > 0.

[h-kataria]

Closing in favor of #4514 , @skateman was working on same task.