Updates for phoenix 1.7 and phoenix_html 4.1

[llaisdy]

Thank you for creating the Contex library, and this contex-samples demo site!

I tried it out recently and it didn't work with Phoenix 1.7 -- hence this PR.

In mix.* and lib/ I have made minimal changes to make the site compatible with Phoenix 1.7. These changes are all in the version updates and lib commits.

The other changes are mostly up-to-date boiler plate -- I actually created a fresh project and copied contex-samples content over.

This is a big PR with a lot of changes -- so, apologies -- but the whole package seems necessary to shift the project as a whole to current versions.

One caveat: on my (ageing) Mac, I had to add a Node option to make one of Node's app play nicely with the Mac's ssl library:

$ export NODE_OPTIONS=--openssl-legacy-provider

Comments welcome!

[mindok]

Thanks @llaisdy - I'll have a good look through and test. It may take me a few days to get to it - I need to test reasonably well as it deploys to the "marketing site" (ha-ha!). Really appreciate you working through this - thanks!

[llaisdy]

It was a very interesting exercise! Sorry it's such a huge PR, I've tried to make the commits logical at least.

Let me know if you find any edge cases or breakages and I'll make necessary changes.