Add airflow webserver URL into SLA miss email.

[abhishekbafna]

---

^ Add meaningful description above

This adds the Airflow webserver base URL in the SLA miss notification email. This can be really helpful in cases where multiple airflow clusters are running and it identifies in which cluster it has happened. It also enables users to reach the airflow cluster with a single click from the email itself.

Read the Pull Request Guidelines for more information.
In case of fundamental code change, Airflow Improvement Proposal (AIP) is needed.
In case of a new dependency, check compliance with the ASF 3rd Party License Policy.
In case of backwards incompatible changes please leave a note in UPDATING.md.

[boring-cyborg]

Congratulations on your first Pull Request and welcome to the Apache Airflow community! If you have any issues or are unsure about any anything please check our Contribution Guide (https://github.com/apache/airflow/blob/master/CONTRIBUTING.rst)
Here are some useful points:

• Pay attention to the quality of your code (flake8, pylint and type annotations). Our pre-commits will help you with that.
• In case of a new feature add useful documentation (in docstrings or in docs/ directory). Adding a new operator? Check this short guide Consider adding an example DAG that shows how users should use it.
• Consider using Breeze environment for testing locally, it’s a heavy docker but it ships with a working Airflow and a lot of integrations.
• Be patient and persistent. It might take some time to get a review or get the final approval from Committers.
• Please follow ASF Code of Conduct for all communication including (but not limited to) comments on Pull Requests, Mailing list and Slack.
• Be sure to read the Airflow Coding style.
  Apache Airflow is a community-driven project and together we are making it better 🚀.
  In case of doubts contact the developers at:
  Mailing List: dev@airflow.apache.org
  Slack: https://s.apache.org/airflow-slack

[potiuk]

Can we see an example screenshot of such an email generated ?

[abhishekbafna]

Can we see an example screenshot of such an email generated ?

Thank you for the review Jarek. Please find the screenshot of the email below.

PS: I have tested this on Airflow 1.10.4+ which we use internally. It's the same change for both the current master and 1.10.4 version.

[boring-cyborg]

Awesome work, congrats on your first merged pull request!

[potiuk]

Nice!

[abhishekbafna]

Thank you Jarek, I couldn't be happier.