Add plaintext definition tooltips for colored labels

[kokerinks]

Summary:

Enhancement: add tooltips to colored labels on CATcher which shows a plaintext version of the label definition

Changes Made:

• Displays plaintext-formatted tooltip for colored labels on:
  • Main page
  • View/Create issues page

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.