Standardize spelling of "Big Bang" and "bigbang" across all help dialogs

Following a conversation on IL4 Mattermost, the Tools team has decided to standardize the spelling of Big Bang (the product) across bbctl has follows:

• Big Bang wherever presented to the end user (in help dialogs, docs, etc)
• bigbang in code references

This change should help our docs feel more cohesive and reduce confusion when referring to the product or a piece of code.

This ticket should go through and confirm / change any outliers. This should be completed at the end of the epic as a final pass.