Upgrade to Gatsby v3

[cainmagi]

First, let me thank the author for such an awesome theme.

The last time this project is updated is 2 years ago. Now some of the features in this project are out-of-date. So I make such modifications:

• The Gatsby used by this package has been bumped from 2 to 3.
• The sass version has been bumped to sass 2.
• Drop some deprecated sass usages.
• Drop javascript:; deprecated by React.

The above modifications will upgrade this project to Gatsby v3, and remove all warnings caused by deprecations.

If the author is willing to accept this PR, for the compatibility issue, I suggest that the author should

• add a branch named as gatsby-v3 and merge my PR to that one,
• or backup the current master branch as gatsby-v2 and merge my PR to master.

Thank you!

[cainmagi]

It seems that the checks fail because your development environment is also out-of-date. Please check my modifications with nodejs v14+ (and yarn v1.22+ if needed).