Add stub service worker so users can install on desktop with Chrome

[daleharvey]

As discussed in #11646

---

Fixes #11646

[daleharvey]

So the sw.js is just a stub file, it needs a fetch event listener or chrome will not consider it installable.

We could of course actually cache all the static assets, it would have a far bigger impact on performance than any of the reducing bundle size type optimisations, however that seemed like a lot of work given that its been mentioned riot may not always support the web (#11646 (comment))

At the least this makes it installable and adding the actual caching could be done as a follow up

[turt2live]

https://github.com/vector-im/riot-web/pull/11774/files#r363474651

[daleharvey]

Updated the script tag to move it to line 44 as suggested, also here is what it looks like installed

[turt2live]

@daleharvey , @jryans will be taking this one on.

dismissing because it's not really relevant

[jryans]

Thanks for the contribution @daleharvey! I think this looks good technically, and we're going to give this a try in an experimental capacity, under the assumption that installable PWAs should just work the same as the regular web app.

As one last step, could add your sign off to the PR as described in CONTRIBUTING.rst? Once you've done that, we should be ready to merge. You can just add a comment with the sign off line if you like.

[jryans]

its been mentioned riot may not always support the web (#11646 (comment))

Just to clarify for others, Riot is committed to supporting both web and desktop for the long term. See #11646 (comment) for more detail.

[daleharvey]

Awesome thanks, I added the Signed-off to the commit

[jryans]

Thanks for working on this! 😁 It should show up on https://riot.im/develop in a few minutes and in the next release in a few weeks.