Add doc for link_to

[ci skip]
Merge branch 'doc-link-to'
Add button doc

[ci skip]
Merge branch 'add-button-doc'
Add modal documentation
Merge branch 'better-modal-doc'
Add icon documentation
Merge branch 'icon-doc'
Deprecated glyphicon in favor of icon

Because of Semantic Versioning, glyphicon will still work until v2.
Merge branch 'remove-glyphicon-doc'
Add progress_bar documentation
Merge branch 'add-progress-bar-doc'
Pass `id` param to dropdown subcomponents

In a dropdown, the ID of the button match the 'data-target' of the `<ul>`
element in order for the the dropdown "toggle" to work.

Before this commit, a random ID was generated for each dropdown.
After this commit, a specific ID can be assigned by passing the `:id`
parameter to the `dropdown` helper.
Add dropdown documentation
Merge branch 'add-dropdown-doc'
Add panel documentation
Merge branch 'add-panel-doc'
Add panel_row documentation
No need to overload `panel_row`

The set of panels to include in a `panel_row` cannot possibly be
passed as a string.
Merge branch 'add-panel-row-doc'