Update issue templates

[liamsi]

We currently re-use the templates inherited from tendermint. Now that the team is growing and the repository is gaining some traction, we should update these according to our needs.

Same with the security policy: https://github.com/lazyledger/lazyledger-core/security/policy

ref: #120

[liamsi]

Reprioritized as the quality of the issues are key to communicate what is going on. Good issue templates can facilitate well-written and actionable issues.

[liamsi]

I think these issue templates should exist:

• Proposal (could also be a discussion sometimes)
• Write ADR for certain aspect
• Implement FooBar
  • sub-task of foo-bar
• Bug
• Question (could also be a discussion in most cases)
• (maybe) Tracking issue

[liamsi]

IMO tracking issues should be created manually depending on the scope of the work and questions can simply be asked as part of a discussion or QA discussion board. closing as done.