Fix merged table rendering in Python env

[rmccar]

What is the context of this PR?

Fixes: ONSDESYS-653

This PR fixes some bugs in the table macro related to some logic in that macro that doesnt work correctly in a python/jinja2 environment.
The first change is to the way the ons-table--has-rowspan is added to the <table> element. In jinja2 any variable set inside a for loop are only accessible in that for loop so the changes to the hasRowSpan var were inaccessible from outside of the loop so it was always false when it came to adding the class. This had to be refactored to add the class inside the loop instead of setting the var. This fixed the spacing and border issues.
The second change fixes the way keys are accessed and set within object in a jinja2 env. Without the quotes around ths this was being set as undefined and therefore in the loop {% for th in headerCols.ths %} this key was inaccessible so the titles were never being set.

How to review this PR

• spin up the design-system-python-flask-demo repo and see that the column titles are missing, the spacing is off on the second column and the bottom border does not span the width of the table in the example-table-merge-cells example
• manually copy the table macro from this branch into the design-system-python-flask-demo repo
• remove the load-design-system-templates step from the run command in the make file
• spin up that repo again and see that these issues are resolved
• do the same steps in the DS repo to make sure nothing is broken or there aren't any unintended changes there

Checklist

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