Add bootswatch theme specific tip box styles

[ang-zeyu]

What is the purpose of this pull request? (put "X" next to an item, remove the rest)

• [x] Enhancement to an existing feature

Resolves #962

Accompanying pull request to MarkBind/vue-strap#118

What is the rationale for this request?
To add tweak tip box heading styling and add theme specific styles for said headings, as good looking headings is difficult to achieve on all themes with generic styling.

What changes did you make? (Give an overview)

• Added bootswatch theme specific styles for tip boxes
• Some code to copy this additional stylesheet 'layer' when building the site

Preview (uses theme's alert-xx color)

Preview (theme-specific color):

Provide some example code that this change will affect:

// Under site.js, does the same as getBootswatchThemePath
function getExtraBootswatchThemePath(theme) {
  return path.join(__dirname, 'lib', 'bootswatch', 'src', theme, 'bootstrap-markbind.css');
}

// Under site.js SUPPORTED_THEMES_EXTRA_PATHS constant
...
'bootswatch-cerulean': getExtraBootswatchThemePath('cerulean'),
'bootswatch-cosmo': getExtraBootswatchThemePath('cosmo'),
'bootswatch-flatly': getExtraBootswatchThemePath('flatly'),
...

Is there anything you'd like reviewers to focus on?
The implementation of theme specific styles ( #997 ),
mainly whether bootswatch should be patched directly.

Testing instructions:

1. Build the dist from Enhance box component heading vue-strap#118
2. Use the built dist and execute markbind serve -d to launch the template page with all the variants of the tip boxes.
3. Use the style switcher component under heading 2 to switch between the themes quickly for testing ( will be removed from the PR if the styles are ok )

1. npm run updatetest should pass ( number of assets generated unit test is changed, and expected test files )

Proposed commit message: (wrap lines at 72 characters)
Add bootswatch theme specific tip box styles

[acjh]

Thank you for the attempt, but we really shouldn't do this. It's a lot of effort for very little value.
My thoughts on styling: #386 (comment), #386 (comment), MarkBind/vue-strap#124 (comment)

[ang-zeyu]

Noted 👍

Hmm, shall we go with the barebones approach mentioned in #962 then? @damithc @acjh

It's plain, but still looks good imo, and dosen't require theme specific styling
I'll edit the main post with a new gif in a while

[ang-zeyu]

As discussed with @damithc, we'll simply go with the alert-theme-xx's color for easier maintainability.

I'll close this PR and make a new one for posterity