Fix .form-inline docs example

Make sample code match live example
Move <label> out of .input-group into .form-group instead
Change field from Email to Username to make it less redundant, slightly more realistic
Add <label> for Username field for consistency
Fixes #15118
Pin npm-shrinkwrap to v5.0.0 so that it works with npm 2.x.

Fixes #15105.
Merge pull request #15120 from twbs/fix-forms-doc

Fix .form-inline docs example
Use a 40x40 image for the `navbar` brand image example.

Saves 3.4KB.
give all docs callouts IDs

[skip sauce]
Fix a small grammar mistake.

[ci skip]
Merge pull request #15121 from twbs/npm-shrinkwrap@5.0.0

Pin npm-shrinkwrap to v5.0.0 so that it works with npm 2.x.
Merge pull request #15093 from twbs/id-callouts

Give all docs callouts IDs
[skip sauce]
bump remarkable devDependency to ~1.4.1

[skip sauce]
Merge pull request #15122 from twbs/bump-remarkable

bump remarkable devDependency to ~1.4.1
clarify that Bower is one among several installation methods