Standardize variable syntax

[jamos-tay]

What is the purpose of this pull request? (put "X" next to an item, remove the rest)

• [X] Other, please explain: Refactoring

Fixes #710

What is the rationale for this request?

Two different variable syntaxes are used, <span id=...> for include and global variables, <variable name=...> for page variables. For standardization we should only use one.

What changes did you make? (Give an overview)

Change all variables to use <variable name=...> syntax.

Testing instructions:

I left compatibility for <span id=...> - prevents old sites from breaking. Should it be removed?

Can confirm something like this works:

<span id="a">1</span>
<variable name="b">2</variable>

[yamgent]

I left compatibility for <span id=...> - prevents old sites from breaking. Should it be removed?

I think it should be removed eventually, don't see the value in the old syntax since it is hard to differentiate it from actual <span>s.

We will deprecate this feature first, then remove it by the next big release as per standard practice.

[yamgent]

Looked through the updated logic, and also skimmed around the documentation a bit to see whether all necessary replacements are done, seems generally fine to me 👍.

[jamos-tay]

Rebased to solve merge conflict

[yamgent]

Proposed commit message:

Standardize variable declaration (#743)

We use <span> to declare variables in include and global variables.

<span> variables are difficult to differentiate from actual <span>
tag uses. Furthermore, in places where this differentiation is
important (e.g. pages where <span>s are extremely common), we are
forced to use <variable> tags, rather than <span> tags. This results in
two different (and confusing) ways of declaring variables.

Let's standardize all variable declaration to use <variable> instead
of <span>.

The processing of <span> variables are currently left intact, but
they are deprecated and should be removed in a subsequent release.