Fix Bootstrap 4 help-text components

- Also fix typo in template
- Also remove no-longer-used "bootstrap-datetime-picker" from
  "package.json"

package-lock.json

       "integrity": "sha512-o9ppKQioXGqhw8Z7mah6KdTYpNQY//tipnkxppWhPbiSWdD+1raYsnhwEZjkTHYbGee4cVQ0Rx65EhOY/HNLcQ==",
       "dev": true
     },
-    "bootstrap-datetime-picker": {
-      "version": "2.4.4",
-      "resolved": "https://registry.npmjs.org/bootstrap-datetime-picker/-/bootstrap-datetime-picker-2.4.4.tgz",
-      "integrity": "sha1-t8/oQ6PG0iHAMpPJpGSkzDXXOWg=",
-      "dev": true
-    },
     "brace-expansion": {
       "version": "1.1.11",
       "resolved": "https://registry.npmjs.org/brace-expansion/-/brace-expansion-1.1.11.tgz",

package.json

   "devDependencies": {
     "@fortawesome/fontawesome-free": "^5.15.1",
     "bootstrap": "^4.5.3",
-    "bootstrap-datetime-picker": "^2.4.4",
     "eslint": "^7.14.0",
     "extend": ">=3.0.2",
     "gulp": "^4.0.2",

src/oscar/templates/oscar/dashboard/partials/form_field.html

 
                     {% block help_text %}
                         {% if field.help_text %}
-                            <span class='form-text'>
+                            <small class="form-text text-muted">
                                 {# We allow HTML within form help fields #}
                                 {{ field.help_text|safe }}
-                            </span>
+                            </small>
                         {% endif %}
                     {% endblock %}
                 </div>

src/oscar/templates/oscar/dashboard/partials/form_fields.html

 {% if form.non_field_errors %}
     {% for error in form.non_field_errors %}
         <div class="alert alert-danger form-group">
-            <span class="help-inline"><<i class="fas fa-exclamation"></i> {{ error }}</span>
+            <span class="help-inline"><i class="fas fa-exclamation"></i> {{ error }}</span>
         </div>
     {% endfor %}
 {% endif %}

src/oscar/templates/oscar/dashboard/reviews/review_update.html

                                         </span>
                                     {% endfor %}
                                     {% if field.help_text %}
-                                        <span class='form-text'>
+                                        <small class="form-text text-muted">
                                             {{ field.help_text }}
-                                        </span>
+                                        </small>
                                     {% endif %}
                                 </div>
                             {% endif %}

src/oscar/templates/oscar/partials/form_field.html

 
                     {% block help_text %}
                         {% if field.help_text %}
-                            <span class='form-text'>
+                            <small class="form-text text-muted">
                                 {# We allow HTML within form help fields #}
                                 {{ field.help_text|safe }}
-                            </span>
+                            </small>
                         {% endif %}
                     {% endblock %}
                 </div>