Clean up .gitignore

From https://git-scm.com/docs/gitignore:
> Which file to place a pattern in depends on how the pattern is meant to be used.
> * Patterns which should be version-controlled and distributed to other repositories via clone (i.e., files that all developers will want to ignore) should go into a `.gitignore` file.
> * Patterns which are specific to a particular repository but which do not need to be shared with other related repositories (e.g., auxiliary files that live inside the repository but are specific to one user’s workflow) should go into the `$GIT_DIR/info/exclude` file.

That makes it seem like most of the patterns in our `.gitignore` should not be there
https://github.com/250MHz/bpmail/blob/3fa8cd92062135e964c257c5a9f938d8442fcdf9/.gitignore

Remove the patterns that don't fit the first bullet point above.

Wiki's development guide should also advise people to use `$GIT_DIR/info/exclude`.