Make navbar and footer more responsive

[davidwilby]

• For xs and sm screens doesn't display the footer, instead showing in the navbar's hamburger menu.
• Adds bottom padding to body for md and larger screens to avoid overlap with footer.
• Moves the user account drop down out of the <b-collapse> section so it is shown more prominently for smaller screens - I appreciate this might not be to everyone's taste, so can remove this commit if needed.

Screencast.from.13-04-23.15.17.19.webm

Resolves #331

[github-actions]

Jest Coverage

File	% Stmts	% Branch	% Funcs	% Lines	Uncovered Line #s
All files	80	77.37	74.19	79.92

File	% Stmts	% Branch	% Funcs	% Lines	Uncovered Line #s
All files	80	77.37	74.19	79.92
_src/components	77.14	77.35	85.71	77.14
_ AnnotationRenderer.vue	76.23	77.35	84	76.23	126,141-142,196,219,240,247-261,269,275-285,319
_ MarkdownEditor.vue	100	100	100	100
_ MarkdownRenderer.vue	100	100	100	100
_src/components/annotation	80.95	86.66	82.35	80.95
_ CheckboxInput.vue	85.71	100	100	85.71	19
_ HtmlDisplay.vue	80	100	100	80	26-30
_ InputErrorDisplay.vue	100	0	100	100	19-25
_ RadioInput.vue	85.71	100	100	85.71	19
_ SelectorInput.vue	71.42	100	66.66	71.42	20,27
_ TextInput.vue	75	100	66.66	75	26
_ TextareaInput.vue	66.66	100	50	66.66	20
_src/enum	100	100	100	100
_ DocumentTypes.js	100	100	100	100
_src/jrpc	90.32	83.33	75	90.32
_ index.js	90.32	83.33	75	90.32	29,38-39
_src/utils	77.77	74.6	38.46	77.46
_ annotations.js	96.55	81.25	100	96.42	35
_ dict.js	100	100	100	100
_ index.js	55.88	54.54	11.11	55.88	9-14,28-93
_tests/unit	100	100	100	100
_ globalVue.js	100	100	100	100

[twinkarma]

LGTM!