use the same username in all guides

[luto]

We currently use a different username for each guide (e.g. wordpress for wordpress, ghost for ghost). This increases the work we have with maintaining guides, makes it hard to copy code around and makes it impossible to reuse snippets without causing confusion. Additionally, we used female-sounding names in the past to make a statement, which we can't do with project-names as usernames.

• make up a single username to use in all guides
• change all existing guides to match
• document this in our style guide