Remove the dependency on Zenhub

[daira]

Proposal: allow dependencies to be expressed in the initial comment of a ticket, using the following syntax (case-insensitive).

Blocked-by: <list of [[org/]repo]#number>
Blocks: <list of [[org/]repo]#number>

Note that unlike Zenhub, this doesn't prevent creating cycles in the graph. What I suggest is to render a general directed graph rather than a DAG, but colour the arrows that are part of a cycle in red. This makes it clear that cycles are an error and makes them easy to spot in order to fix them manually.

To detect cycles in a directed graph, use any linear-time algorithm for finding strongly connected components, e.g. Tarjan's algorithm, Kosaraju's algorithm, or the Path-based strong components algorithm.

[daira]

This is essential to fully expose the DAG publically and remove one of ECC's information asymmetries.