docs: update calendar to match the 18.2 plan with the one announced

Additionally, I've extended the 18.1 cycle by one more release,
tentatively assigned to Dylan, due to the ~2 weeks delay for 18.2.

Cc: Dylan Baker <dylan.c.baker@intel.com>
Cc: Juan A. Suarez <jasuarez@igalia.com>
Cc: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Andres Gomez <agomez@igalia.com>
Acked-by: Dylan Baker <dylan@pnwbakers.com>
Reviewed-by: Juan A. Suarez <jasuarez@igalia.com>
Acked-by: Emil Velikov <emil.velikov@collabora.com>

docs/release-calendar.html

@@ -39,7 +39,7 @@ if you'd like to nominate a patch in the next stable release.
 <th>Notes</th>
 </tr>
 <tr>
-<td rowspan="3">18.1</td>
+<td rowspan="4">18.1</td>
 <td>2018-07-27</td>
 <td>18.1.5</td>
 <td>Dylan Baker</td>
@@ -55,29 +55,35 @@ if you'd like to nominate a patch in the next stable release.
 <td>2018-08-24</td>
 <td>18.1.7</td>
 <td>Dylan Baker</td>
+<td></td>
+</tr>
+<tr>
+<td>2018-09-07</td>
+<td>18.1.8</td>
+<td>Dylan Baker</td>
 <td>Last planned 18.1.x release</td>
 </tr>
 <tr>
 <td rowspan="4">18.2</td>
-<td>2018-07-20</td>
+<td>2018-08-01</td>
 <td>18.2.0rc1</td>
 <td>Andres Gomez</td>
 <td></td>
 </tr>
 <tr>
-<td>2018-07-27</td>
+<td>2018-08-08</td>
 <td>18.2.0rc2</td>
 <td>Andres Gomez</td>
 <td></td>
 </tr>
 <tr>
-<td>2018-08-03</td>
+<td>2018-08-15</td>
 <td>18.2.0rc3</td>
 <td>Andres Gomez</td>
 <td></td>
 </tr>
 <tr>
-<td>2018-08-10</td>
+<td>2018-08-22</td>
 <td>18.2.0rc4</td>
 <td>Andres Gomez</td>
 <td>Last planned RC/Final release</td>