Add support for subresource integrity

[mrmonday]

Add support for generating subresource integrity hashes at build time

• Add an sri feature, which depends on the ssri crate
• Generate sha256, sha384, and sha512 SRI hashes for statics
• Update examples to use SRI

[kaj]

Hi! Thanks for this contribution! It seems like a good thing overall, I'll try to take good look at the details tomorrow (I'm a bit too tired today).

[kaj]

Now I've taken a better look at this. Sorry it took closer to a week than a day ... I have mainly one question:

Should a template author have to specify which kind integrity hash should be used for each place a resource is linked? Seems to me a project would probably like to decide on one hash size / algorithm, and if / when the time to change it comes, it would be nicer to change it in one place in the project, rather than in every link or script tag in every template.

So, instead of including three separate fields with diffent integrity hashes, I think it would be better to only have one field (named integrity without suffix) containing one of the hashes. We could choose any of the hash algorithms as a default, and add a method to be called from the build script that can change the hash algorithm (that addition could be either as part of this PR, or left as room for future improvement after just hardcoding one of the hashes in this PR).

[mrmonday]

If this ends up as a single method, it needs to be possible to specify multiple hash algorithms, as well as just one (eg. integrity="<hash1> <hash2> <hash3>" is valid).

I opted to go the old fashioned route of serving static resources from the file system for what I'm working on, so probably won't do any more work on this PR - feel free to push against the branch if you want to continue work on it.

[vbrandl]

I might be willing to pick this one up, rebase and make the used hash algorithm globally configurable in build.rs.

But before doing that: Does subresource integrity really make sense when everything is served from the same server? I know it makes sense when using external resources to prevent a CDN/external hosting provider from maliciously changing the file. But for resources that are served from the same server as the file that uses the resource, if an attacker has access to the server, they can change both the resource and the SRI hash. What's the attack vector, SRI for local resources protects from?