cleanup some pre css inheritance

[minrk]

We had various overrides of the bootstrap defaults for font-size and line-height every time we made a pre tag, and then some overrides of our overrides when we wanted something slightly different.

This moves the most basic overrides to apply to all pre tags, reducing the number of downstream changes we need.

Should hopefully replace the CSS changes in #5488

[minrk]

Related question (especially for @ellisonbg): Do we ever want pre tags to have line-height other than @code_line_height? If not, we should probably move @code_line_height to the base variables.less, and define the default line-height for pre as @code_line_height instead of inherit.

As far as I can find, all pre tags expect this line height (input, output, prompt, pager - any others?), so it makes more sense for these to be the pre defaults rather than setting them repeatedly all over the place. The same might go for @code_padding, but we seem to apply the padding sometimes to containers instead of pre, so maybe not for that one.

[fperez]

I just tested before and after this PR merge, and I'm actually OK for my PyCon talk: my CSS tweaks don't seem to be affected by any of this, so I'm not under any pressure to worry about it.

Feel free to review and merge at leisure, it won't bug me. Thanks though!!