Consider syncing capability checks

[tw2113]

We check for manage_options for the admin screen, but install_plugins for the actual action of running the switching.

Perhaps make install_plugins the requirement to even see the admin screen?

Original issue: https://wordpress.org/support/topic/does-not-respect-capabilities-canyou-please-fix-asap/#post-12748575

[sc0ttkclark]

• manage_options is used for the management menu page: https://github.com/WebDevStudios/taxonomy-switcher/blob/52b0fea30aeab17dee1ba063a5d412f565f88277/Taxonomy_Switcher_UI.php#L87
• install_plugins is used for running the conversion: https://github.com/WebDevStudios/taxonomy-switcher/blob/52b0fea30aeab17dee1ba063a5d412f565f88277/taxonomy-switcher.php#L51

I suggest manage_options should be the capability used for both.