Add plaintext tooltips to labels

Summary:

I was inspired by issue #886 , to add tooltips for labels (Severity, Bug types and Response types).

I then extended this to include the colored labels as well (as shown under Changes Made)

Attempted to get markdown-supported tooltips for CATcher (to support the definitions written in MD), tried dependencies like primeng, ng-bootstrap, tippy.js etc. but most are not supported by ng13. Hence, I settled for a function to convert the markdown definitions into a readable plaintext form.

FOOTNOTE: If this were to be pushed into CATcher, I think only the tooltips on colored labels might be relevant for the final product, since the dropdown menu already includes an info icon with the definitions in Markdown. However, I have included both changes for this PR for a show of working.

Changes Made:

Add tooltips for labels in dropdown menu(Severity, Bug types and Response types)
Aside from the change suggested in the issue, the tooltips were also added to these colored labels:

Proposed Commit Message:

Add tooltips for definitions of dropdown menu labels and colored labels

For users who are unfamiliar with the definition of labels like 
`severity.Medium` or `type.FeatureFlaw`, they may need easier access to
them (especially for PE users, who rely on these definitions to ensure 
that their bug reports are accurate.)

Hence, this PR adds tooltips to these labels to display the
plaintext definition of the label when hovered over.

CATcher-org / CATcher