Contributed by @gjacob24 of DCC DMPonline.
    Changes:
    - Adjusts the layout and displays the 'optional plan components' and 'select phase to download' side by side for multi-phase plans.

    Co-authored-by: gjacob24 <gjacob@ed.ac.uk>

- This changes expand upon commit 87ddbb0

- This change improves the styling/positioning between the "Optional plan components" and "Select phase to download" headings.
  - Added "form-control" class to "Optional plan components" heading to match "Select phase to download" stylings
  - Replaced `<legend>` element with a `label_tag`+ `form-label` class to again improve consistency between the two headings.
    - Note, the added `label_tag` doesn't associate with any specific form control, so the first paramater is left empty.

…of-style-of-select-tags-and-plan-download-layout

Additional `form-control` to `form-select` changes & styling changes to "Download settings" subheadings

…ect-tags-and-plan-download-layout

- Commit 72c2519 improved the alignment between the 'Optional plan components' and 'Select phase to download' headings. However, the change from <legend> to an unreferenced label_tag() hurt the accessibility.

- This change puts back the <legend> tag and uses class: 'col-form-label' for both of the aforementioned headings to maintain stylistic consistency between the two elements.
  - `.col-form-label` overrides <legend> styling to maintain consistent alignment with <label> elements.

- Accessibility snapshot of <fieldset> before this fix:
Name: ""
aria-labelledby: Not specified
aria-label: Not specified
From legend: Not specified
title: Not specified
Role: group
Invalid user entry: false

- Accessibility snapshot of <fieldset> after this fix:
Name: "Optional Plan Components"
aria-labelledby: Not specified
aria-label: Not specified
From legend: legend.col-form-label "Optional Plan Components"
title: Not specified
Role: group
Invalid user entry: false
Labeled by: legend.col-form-label