Add empty `alt` attribute to img tag with logo (#2035)

* Provide empty alt attribute (alt="") for logo image, because it does not provide information

Fix empty `<img>` when `site_logo` is not assigned

Ref: https://github.com/mmistakes/minimal-mistakes/pull/2026#issuecomment-454809876

Release 4.15.0 💎

Add logo and title customization to the masthead (#2026)

* Add logo and title customization to the masthead

* Adjust config description

* Add test site logo to `/test`

* Document `site.logo` and `site.masthead_title`

* Update CHANGELOG and history

Update CHANGELOG and history

Fix scrollspy on link clicking, and # for top of page (#2023)

Update

Add support to customize `issue-term` (#2022)

Current we hardcoded the `pathname` for `issue-term` when we are using utterances comment provider.

It would be better for let users customize it by setting a `site.comments.utterances.issue_term` in their `_config.yml`.

Update CHANGELOG, history, and `canonical_url` documentation

assign canonical directly if custom value exists (#2021)

Update CHANGELOG and history

Merge pull request #2020 from edemaine/scrollspy

Define $active-color in terms of $primary-color

Use jQuery throttle library

Scrollspy-style updating of hash and TOC highlighting

Update CHANGELOG and history

Clicking TOC links changes hash fragment (#2019)

* Fix smooth scroll breaking back button #1767
* Change URL's hash fragment when clicking on TOC
* Switch from hashchange to popstate event handler

This seems to have better behavior with the Forward button, on page load,
and always scrolls in the right direction.

Close #1767 

Bump version to 4.14.2

Release 4.14.2 💎

Update copyright year