Infoclick: allow for some common Markdown and URL characters inside the infoclick placeholder string

[jacobwod]

#1277 was an attempt to fix Infoclick's {{if}} syntax to also accept = sings inside the attribute value (e.g. HTML links often have equal sings inside the string).

Unfortunately I identified some cases where the fix from #1277 is not enough. One example is such a value:

<p>Some nice copy here!</p>
<p>Take a look at this.</p>
<p>See also: <a href="https://foo.example.com/?u=http%3A%2F%2Fwww.example.com%2F%3F9IAUwj5OWI_FU5zuoHf6hrAb-DCk4A1AHlEqgwcL6mwR16bFDi2c-A-kYWA8DzQ35LPtZb7OWrN43x_GBzv5BGX1VNHX3jAUWi3Uf&__tn__=q&c%5b0%5d=AT0OsszzvOh0d6heT-cUEMZlm4sJZB5UqYJ3G3xIsve42Qjqh0J_tBGP-t1Xay5PHlUeqsVAF7bWA3bgyEushsP1dht2-9BiCr7sqfXnnQ2Oclji4S1Is9TI6BsgEFB5X4m_r310mHVN3I_DYr7QB7q00oxd" target="_self">www.example.com</a></p>

If used inside the {{if}} syntax, the code above will go into the compare branch because a) there is an equal sign inside the string and b) we failed to identify that the equal sign is just a part of an URL.

I think that we've reached the end of the road for the {{if}} solution and we must find another approach.

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We already have the filters funtionality in Infoclick, see docs here (unfortunately, the docs aren't up to date).

I found the undocumented hasValue feature, which would pretty much fit our needs:

Example:
  {someFieldFromDb|hasValue('It has a value', 'Sorry, no value here.')}

The syntax above will correctly match as an placeholder and will be passed to the filters function. But, as soon as we introduce
some Markdown to the string, it won't match as a placeholder:

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Proposals

Part 1

Let's extend the regex that matches placeholder to also include (some) common Markdown characters.

Simply adding a * to the regex fixes the first case:

But to fix more complex cases, we must add more chars:

Part 2

Since this will probably affect existing setups, we should make this a setting (in admin/appConfig/whatever) so that it's possible for admins to change, verify, test, change back and so on, until all configuration is converted and confirmed working.

[jacobwod]

The solution in 93d2e12 had one limitation: it matched the closing conditional tag of {{/if}}, effectively breaking the if-conditions. I couldn't live with that so I worked on another solution.

I ended up with completly refining the overly complicated regex used for matching placeholder. The idea is to use a broad regex that matches anything between { and }. This won't be good, of course, as our {{if ..}} will also get matched:

/{[^}]*}/g 

So I came up with an additional, string-based, filtering that takes care of that and eliminates unwanted matches:

this.markdown.match(this.placeholderMatchingRegex) || [])
        .map((match) => match.replace("{{if ", ""))
        .filter((match) => match !== "{{/if}")
        .forEach((placeholder) => {
           // Do the usual stuff with placeholders

I now seem to get the wanted results, but this will need testing to ensure broad compatibility.