- Adds `searchBy` util in common utils
- Adds placeholder `SidebarItemEpicsSelect`

Changes migration and all other places the attribute is used

This is one of many MRs opened in order to improve the overall
internationalisation of the GitLab codebase.

i18n documentation
https://docs.gitlab.com/ee/development/i18n/externalization.html

This change brings new Scoped labels to Issue board as well.
With the last change, this was missed.

Add time tracking progress bar to Issue Boards sidebar.

Reduced the technical debt around our JS flash function by making it a
module that is imported rather than relying on the global function.
The global function still exists mainly for technical debt with how
some requests are being completed, but new JS should import the module
directly.

Also reduces some tech debt in the file by removing the need for jQuery.
Instead Flash is now 100% vanilla JS.

Introduced in
https://gitlab.com/gitlab-org/gitlab-ce/commit/adc59353fa48c5e433b80efc1698c0b12e121968#852fdbe4145be33eca58d30d8c2f4a6623ef8602_30_30

Only usage removed in
https://gitlab.com/gitlab-org/gitlab-ce/commit/de9e559d4495560095adfa03ea6489ea51f6f2f4#e22df3c6b3631af90cde45bbb974ddce72aefab7_21_21

Also implemented same behavior on the Issue Boards assignee block.
Fixed the default state for search/filter dropdown for issuables.

Fixed users href path being incorrect