Tweaks to welcome email and signatures across all html emails

[bjester]

Summary

Description of the change(s) you made

• Addresses Additional tweaks to welcome email formatting and links #3713 by removing <br> and removing production override of SITE_ID
• Updates all HTML emails that were using a data URI for the LE logo in the email signature

Manual verification steps performed

1. Overrode activation email condition in views/users.py
2. Created an account and activated it
3. Copied the HTML into a file, unescaped newlines and quotes

---

Reviewer guidance

How can a reviewer test these changes?

We should re-test this on hotfixes after merging, since email clients can be finicky

References

Fixes #3713

Comments

---

Contributor's Checklist

PR process:

• If this is an important user-facing change, PR or related issue the CHANGELOG label been added to this PR. Note: items with this label will be added to the CHANGELOG at a later time
• If this includes an internal dependency change, a link to the diff is provided
• The docs label has been added if this introduces a change that needs to be updated in the user docs?
• If any Python requirements have changed, the updated requirements.txt files also included in this PR
• Opportunities for using Google Analytics here are noted
• Migrations are safe for a large db

Studio-specifc:

• All user-facing strings are translated properly
• The notranslate class been added to elements that shouldn't be translated by Google Chrome's automatic translation feature (e.g. icons, user-generated text)
• All UI components are LTR and RTL compliant
• Views are organized into pages, components, and layouts directories as described in the docs
• Users' storage used is recalculated properly on any changes to main tree files
• If there new ways this uses user data that needs to be factored into our Privacy Policy, it has been noted.

Testing:

• Code is clean and well-commented
• Contributor has fully tested the PR manually
• If there are any front-end changes, before/after screenshots are included
• Critical user journeys are covered by Gherkin stories
• Any new interactions have been added to the QA Sheet
• Critical and brittle code paths are covered by unit tests

---

Reviewer's Checklist

This section is for reviewers to fill out.

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md

[LianaHarris360]

The updated code makes sense to me. More testing on hotfixes will validate the changes!