Fix web_gui build

[lalten]

The web interface is currently broken on master and won't build.

This PR:

• Fixes the missing SafeHtml type
• Reverts the (partial?) update to Angular 10
• Reverts a change in a spec file to be compatible with Angular 4 again
• Replaces optimization in webpack with Terser (was: built-in UglifyJS)
• Adds a new build of the web_gui (this commit could be dropped)

Fixes #1039

---

This change is 

[lalten]

@dbhatman This fixes the web_gui and would allow the changes in #1036.

[lalten]

@glados-verma @dbhatman would be happy to get your eyes on this.
Note that with a788f96 I added a CI stage to assert the web_gui build continues to work in the future

[glados-verma]

Thanks for your excellent work @lalten !

Note for any future readers - we still need to downgrade the Node version (e.g. via the n command) to v9 in order to build this (PR #1024 might help us use newer Node versions). Additionally the instructions in CONTRIBUTDING.md are outdated; check the README.md in the web_gui directory (I'll fix the docs in a separate PR).