`HOW_WE_USE_GITHUB.md`: Updates to working with issues, labeling and other general updates

[kalawac]

Added section to How_We_Use_Github.md on Working on Tickets; updated information on labels to highlight mandatory labels, include information on label descriptions and provide a general overview of the three mandatory label categories; separated out information on the process of issue sorting from information on what information sorting actually is; added information on spikes; changed infra --> infrastructure, in line with current repo name

Description

Resolves #677

• Added instructions on how to assign yourself to a ticket you are actively reviewing (see section Working on Tickets, beginning on line 293)
• Added Instructions for how to use the source::* labels (see section What labels are required for each issue?, beginning on line 160)
• Reviewed provided templates for answering an issue: no change made
• Changed references to conda/infra to conda/infrastructure (current repo name)
• Added information to section on spikes

[kenodegard]

@jezdez good call on conforming to using issue and not ticket

I remember struggling with this since its weird talking about a feature request as an issue or phrasing things like "if you have an issue, open an issue"