Fix links and file name in emails for patchset-level comments

Currently, emails that include patchset-level comments will have
links to the file /PATCHSET_LEVEL. This is changed in such way
that all of those links will now link to the comments tab.

Also, the name of the file in the email was /PATCHSET_LEVEL, but now
it is "Patchset".

There are currently no tests about links being valid, simply because
it requires a brute-force string comparison. In my opinion, such a
test won't add much value anyway.

There is now a test about patchset level email notifications, that
confirms that the name /PATCHSET_LEVEL was changed to Patchset Level,
and also confirms that any links involving "/PATCHSET_LEVEL" were
omitted.

Change-Id: I08b6bcaff618ce4a645b1de936339f31d0fe0b15

polygerrit-ui/app/constants/constants.js

diff --git a/polygerrit-ui/app/constants/constants.js b/polygerrit-ui/app/constants/constants.js
index 74ebebd..ff34442 100644
--- a/polygerrit-ui/app/constants/constants.js
+++ b/polygerrit-ui/app/constants/constants.js
@@ -21,6 +21,9 @@
  */
 export const PrimaryTab = {
   FILES: 'files',
+  /**
+   * When renaming this, the links in UrlFormatter must be updated.
+   */
   COMMENT_THREADS: 'comments',
   FINDINGS: 'findings',
 };