Minor improvements

[rafaelcamargo]

Here are some minor but important improvements that are very easy to do:

• Add aria-label to Search input. Resolved on Add support for color schemes #236
• Add noreferrer to Pitsby credits link at the bottom of sidebar.
• Add <meta> tag to index.html.
• Handle aria-hidden on sidebar.
• Make component menu items as anchors instead of non-semantical divs.