Previously these were set in two places:

1. the modified styled component
2. bootstrap styles on `.container`

Move all the relevant bootstrap styles into the modified styled
component.

The previous navbar max width looked too small on large screens with
page content extending beyond the navbar width. This common width should
really be defined in one place, but that's not trivial as described in
the TODO comment.

Future-proofing for Bootstrap v4 change to use flexbox.

It's not clear to me where this bootstrap.css file came from, but it
seems to be roughly a small subset of the bootstrap.css file
distributed¹ in v3.4.1. This commit replaces it with the
bootstrap-grid.css file distributed in v4.6.2.

Bootstrap v4 introduced more precise breakpoints and shifted the
xs/sm/md/lg classes one level down from the previous version.² Usage has
been adjusted accordingly.

¹ https://getbootstrap.com/docs/3.4/getting-started/#download
² https://getbootstrap.com/docs/4.0/migration/#responsive-utilities

… updating to Bootstrap v4

These seem to be more appropriate. Not sure why these weren't necessary
before.