Revamp theme and side panel for Pull Request Details Page

[sn2912]

What Is This Change?

This PR is a preview of visual refresh for the plugin. The idea is to keep the Ui for pull request page close to BitBucket UI to make it more intuitive.
The changes include

1. Changing the current theme with different colors for section titles and page to a unified background color with some dividers
2. UI Changes around builds section with timestamp
3. UI Changes around reviewers section with a better experience to remove and add reviwer.
4. Task section is moved to side panel and UI changes around it for seamless experience. Replacing old button styles with a better UI experience.

How Has This Been Tested?
Checked by opening several PRs on Bitbucket Cloud and server and tried modifying the sections I have made changes to. Works fine.

How Has This Been Tested?

1. Loom Recording showing changes in details and testing for Bitbucket Cloud
2. Loom Recording for testing changes for Bitbucket DC

Basic checks:

• npm run lint
• npm run test

Advanced checks:

• If Atlassian employee & Bitbucket changes: did you test with DC in mind? See Instructions

Recommendations:

• Update the CHANGELOG if making a user facing change

[bwieger-atlassian-com]

Thank you for the very detailed loom testing on both cloud and datacenter.

The base branch was changed.

[akalia-atlassian]

Looks good to me!