Fix DB table creation on update;check deactivation, deletion and attempted install on < PHP 7.3

```
WordPress database error Table 'local.wp_statichtmloutput_urls' doesn't exist for query TRUNCATE TABLE wp_statichtmloutput_urls made by do_action('toplevel_page_statichtmloutput'), WP_Hook->do_action, WP_Hook->apply_filters, StaticHTMLOutput\Controller::renderOptionsPage, StaticHTMLOutput\Controller->detect_urls, StaticHTMLOutput\CrawlQueue::truncate, QM_DB->query
```
```
WordPress database error Table 'local.wp_statichtmloutput_urls' doesn't exist for query SELECT COUNT(*) FROM wp_statichtmloutput_urls made by do_action('toplevel_page_statichtmloutput'), WP_Hook->do_action, WP_Hook->apply_filters, StaticHTMLOutput\Controller::renderOptionsPage, StaticHTMLOutput\Controller->detect_urls, StaticHTMLOutput\CrawlQueue::truncate, StaticHTMLOutput\CrawlQueue::getTotalCrawlableURLs, QM_DB->query
```
```
PHP Fatal error: Uncaught TypeError: Return value of StaticHTMLOutput\CrawlQueue::getTotalCrawlableURLs() must be of the type int, null returned in /app/public/wp-content/plugins/static-html-output-plugin/src/CrawlQueue.php:86
Stack trace:
#0 /app/public/wp-content/plugins/static-html-output-plugin/src/CrawlQueue.php(99): StaticHTMLOutput\CrawlQueue::getTotalCrawlableURLs()
#1 /app/public/wp-content/plugins/static-html-output-plugin/src/Controller.php(189): StaticHTMLOutput\CrawlQueue::truncate()
#2 /app/public/wp-content/plugins/static-html-output-plugin/src/Controller.php(218): StaticHTMLOutput\Controller->detect_urls()
#3 /app/public/wp-includes/class-wp-hook.php(287): StaticHTMLOutput\Controller::renderOptionsPage('')
#4 /app/public/wp-includes/class-wp-hook.php(311): WP_Hook->apply_filters('', Array)
#5 /app/public/wp-includes/plugin.php(478): WP_Hook->do_action(Array)
#6 /app/public/wp-admin/admin.php(254): do_action('toplevel_page_s...')
#7 {main}
thrown in /app/public/wp-content/plugins/static-html-output-plugin/src/CrawlQueue.php on line 86
```