Change link text on extensions page.

[mkarolin]

Updates extensions store link language on the extensions page. Adds
link to the store even if there are installed extensions.

Fixes brave/brave-browser#2497
Fixes brave/brave-browser#1013

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Verified that these changes build without errors on
  • Windows
  • macOS
  • Linux
• Verified that these changes pass automated tests (npm test brave_unit_tests && npm test brave_browser_tests) on
  • Windows
  • macOS
  • Linux
• Ran git rebase master (if needed).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed.
• Add appropriate QA labels (QA/Yes or QA/No) to include the closed issue in milestone

Test Plan:

• Start Brave with a fresh profile,
• Navigate to brave://extensions,
• Verify that the page shows text You don't have any extensions yet. Would you like to [install some]?,
• Verify that clicking on install some takes you to the Chrome Web Store,
• Add one or more extensions,
• Return to brave://extensions page,
• Verify that the page shows text Looking for even more extensions?,
• Verify that clicking on the text takes you to the Chrome Web Store,
• Return to brave://extensions page,
• Click on Details button for any extension installed from Chrome Web Store,
• On the details page, verify that the Source shows text Web Extensions Store.

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Request a security/privacy review as needed.
• Adequate test coverage exists to prevent regressions
• Verify test plan is specified in PR before merging to source

[bbondy]

yep per @yrliou 's comment.

[yrliou]

LGTM