UI: Use more subtle grey SHA1 labels

With grey SHA1 labels, we should consider having also more subtle strips on
commits list. As current strips blend too much with grey SHA1 labels and top
bar, making hard to distinguish headers from content.

UI: Remove CSS rule making last-commit SHA bold

Current green SHA1 labels are more pronounced than other UI elements attracting
attention as if they were most important thing in the UI, while they are not as
important, especially without real Git client.

Using grey SHA1 labels makes the UI more balanced, less aggressive and lets
user to focus on other content elements.

NOTE: Neither GitHub or Bitbucket uses so heavy pronunciation as Gogs.

This removes remains from old design, that was not cleaned by previous #2068 PR.

Further layout fixes

Otherwise the fix will be overwritten by next _octicons.less update.

This is follow-up for 22b0dfbb35.

Merges repo/sidebar.tmpl with repo/header.tmpl and makes every repo page use
middleware.RepoRef() necessary to display information on this menu.

Repo file list layout & misc fixes

This uses a CSS trick making first th to be relative block with width equal to
first two columns, effectively working around inability to use colspan="2" on
first row that was breaking "fixed-layout" for tables.

Also use grey header for last-commit SHA1 tag.

Translation: Add missing entry for new "code" tab

Just use secondary menu instead custom ".head.meta", which simplifies code.

Also do not display repo URL action when we are in subdirectory or viewing a
file.

We have new tab, but we had no entry. That's why it was showing "code"
(lowercase) as this is text id, where we were expecting properly title cased
"Code" to be shown in English version.

Also add Polish translation "code=Kod".

This is more inline with way GitHub looks like and feels much more natural and
in style with rest of the interface.

Instead using own ellipsis, uses Semantic UI fixed single line table which
effectively applies ellipsis to all overflowing table cells.

NOTE: File list cannot use colspan="2" for 1st "Last commit" elements,
otherwise layout breaks with fixed table.

This is because SHA1 label is used in many other places, not only inside
.repository container.

Semantic UI .icon 1em font-size has priority over .octicon 16px, resulting
octicons rendered at 14px font-size, which is not okay since Octicons are meant
to be shown sizes that are multiples of 16px.