Allow linking between entries

[marvin-roesch]

This introduces "links" between entries, which essentially are a codified version of common relationships between entries. Right now, the following relations are implemented:

• Refers to
• Referred to by
• Answers
• Answered by
• Corrects
• Corrected by
• Contradicts

As the names suggest, all these relations come in reciprocal pairs, except for contradiction, which is its own inverse.

The editing experience looks like this and as you can see it allows for specifying that a link is reciprocal, which automatically generates the corresponding link in another entry:

The above links will be displayed as follows:

Links are fully versioned, so they are subject to the usual review process.

This pull request also contains a simple Docker Compose setup for local testing without having to provision the Vagrant VM.

[argent-codes]

I assume the Entry Links section expands on-demand?

Can you also upload a screenshot of how an existing entry would look like after it has been updated with these? You should be able to use any combination of my Renarin & Voidbinding questions:

• https://wob.coppermind.net/events/3/#e64
• https://wob.coppermind.net/events/76/#e6317
• https://wob.coppermind.net/events/176/#e8461
• https://wob.coppermind.net/events/332/#e9541

[marvin-roesch]

The Entry Links section works like the URL sources one, so new links can be added on demand.

I've added a link between two of your WoBs, which would look like this in the context of other WoBs:

[m-strzelczyk]

@PaleoCrafter sorry I'm digging this up after so long, but if you would rebase this PR I think we can merge it finally... Sorry I left it hanging for so long :<