add VPN link helpers

[wen-2018]

One-line summary

This PR adds VPN link helpers similar to bedrock.

Significant changes and points to review

Issue / Bugzilla link

#456

https://mozilla-hub.atlassian.net/browse/WT-138

Testing

┆Issue is synchronized with this Jira Task
┆Priority: P3
┆Sprint: Backlog

[maureenlholland]

@wen-2018 I think this is still worth doing. Not sure where it fits in your queue but maybe we could try to get this merged next week?

[codecov]

Codecov Report

❌ Patch coverage is 76.34409% with 22 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.22%. Comparing base (7861f64) to head (9fe1f6d).

Files with missing lines	Patch %	Lines
springfield/firefox/templatetags/misc.py	69.86%	22 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #475      +/-   ##
==========================================
- Coverage   79.25%   79.22%   -0.04%     
==========================================
  Files         135      135              
  Lines        8375     8468      +93     
==========================================
+ Hits         6638     6709      +71     
- Misses       1737     1759      +22     

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